Have you tried plugged-in the 50CT PCMCIA floppy drive to 110CT, to see if it tried to boot from the floppy?

Have you got the port expander or dock? Further diagnostics will require a parallel port POST code reader and to use that you'll need the port expander to have a parallel port.

What hard drive have you fitted? Double check that the hard drive is installed correctly, because the system won't POST if the hard drive can't be detected - just tested that on my 100CT. It sticks on 0Dh / 19h - see the "l100mm.pdf" for the codes, though I thought it would freeze on 7Bh which is the actual HDD detection code
The Libretto 100CT can fit 9.5mm drives it looks like.

If you have it apart, check the area around where the RTC battery plugs in, my first 100CT had terrible corrosion there and would also not POST.

sangokushi wrote on Today, 03:22:

Have you tried plugged-in the 50CT PCMCIA floppy drive to 110CT, to see if it tried to boot from the floppy?

Yeah, I did that, and unfortunately (and as expected) it made no difference. My 50CT, on the other hand, boots immediately from my Windows 98 boot disk.

Thermalwrong wrote on Today, 11:33:

Have you got the port expander or dock? Further diagnostics will require a parallel port POST code reader and to use that you'll need the port expander to have a parallel port.

I don't yet own a port replicator.
However, the seller is willing to lend me his.
As soon as I have the port replicator, I'll try your suggestion and get back to you as soon as I have any results.
But I'll be honest, so far everything points to a hardware defect (or a corrupted BIOS).

Thermalwrong wrote on Today, 11:33:

What hard drive have you fitted? Double check that the hard drive is installed correctly, because the system won't POST if the hard drive can't be detected - just tested that on my 100CT. It sticks on 0Dh / 19h - see the "l100mm.pdf" for the codes, though I thought it would freeze on 7Bh which is the actual HDD detection code
The Libretto 100CT can fit 9.5mm drives it looks like.

My hard drive is a Toshiba MK0803MAT, and I've made sure it's seated correctly.
It only has 815 MB, but it works perfectly with my 50CT.
So why shouldn't it work with my 110CT?

Thermalwrong wrote on Today, 11:33:

If you have it apart, check the area around where the RTC battery plugs in, my first 100CT had terrible corrosion there and would also not POST.

I checked this in advance and there are no signs of corrosion whatsoever.

Yeah it's weird that it's not working if there's no evidence of corrosion.
Try removing / reseating the memory expansion module if it has one fitted or try without it.
I do recall that my 100CT has scared me a couple of times with not POSTing but reseating the memory card or hard drive fixed it each time.

Thermalwrong wrote on Today, 18:14:

Yeah it's weird that it's not working if there's no evidence of corrosion.
Try removing / reseating the memory expansion module if it has one fitted or try without it.
I do recall that my 100CT has scared me a couple of times with not POSTing but reseating the memory card or hard drive fixed it each time.

Unfortunately my 110CT does not have the memory expansion module.
If it did, it would've totally been my first step too, to check if it's seated properly.
So if it's actually a problem with the onboard RAM, I'm cooked.

The seller of my Libretto suggested, that I should carefully clean the motherboard with 99.9% isopropyl alcohol or a pcb friendly cleaner.
What do you think? Should I try it, or is it a waste of time?

My Libretto 110ct occasionally refuses to boot without the docking station and an external monitor connected. You may need to get a replacement CMOS battery. Also try holding ESC on power up.

onethirdxcubed wrote on Today, 19:10:

My Libretto 110ct occasionally refuses to boot without the docking station and an external monitor connected. You may need to get a replacement CMOS battery. Also try holding ESC on power up.

I really hope you're right.
I would be more than just happy, if this magically brought my Libretto back to life.
But, as I mentioned, I don't have the docking station yet.
As soon as I do, I'll definitely put your suggestion to the test and get back to you.
And if that doesn't work, I'll get myself a replacement CMOS battery.
I need one anyway, since I definitely want to get another 100CT/110CT in the future if I can't get this one working.
I don't have much experience with the 100CT/110CT, so can someone please tell me what CMOS battery I need to order?

Wilius wrote on Yesterday, 13:49:

Battery ... I discovered that the previous owner had already removed it, and there are no signs of corrosion or leakage.

When you say "removed" do you mean "replaced"? or is there no battery at all now. I've not tried this with my Toshibas, but I have seen systems that wouldn't POST without a good CMOS battery.

If there is a battery in it, do you have a multimeter and can check that it is still good? (see above)

If none of the above helps, the first thing I'd propably do is scope the BIOS ROM select ... you should see lots of accesses (ie: continuous if it never boots)
If it gets "some" selects then stops, that would be an indication of bad RAM (or possibly ROM).
This would at least tell you if the CPU and ROM selects are working - but... you need to have (or know someone with) a scope.

Some of the first things most laptop BIOSs do before even trying to boot are:
1) Test some RAM (look for lots of RAM accesses at reset)
2) Turn on backlight (does it come on?) - but it could be burned out or hardware reset state might come up on - so hard to tell unless you can scope a delay
3) Clear screen (can you see "junk" that clears on reset when you power-on (If no backlight then use flashlight as you mentioned))
4) Initialize keyboard and check for "override" keys (look at keyboard scan lines)
Again, to detect most of these you'd need a scope.

DaveDDS wrote on Today, 21:32:

When you say "removed" do you mean "replaced"? or is there no battery at all now. I've not tried this with my Toshibas, but I have seen systems that wouldn't POST without a good CMOS battery.

If there is a battery in it, do you have a multimeter and can check that it is still good? (see above)

The battery has been physically removed.
I'm aware that some laptops won't work without a CMOS battery, but I seem to recall that the Libretto 100CT/110CT can boot without it.
Unfortunately, I can't remember where I read that.
Perhaps I'm just imagining it.

DaveDDS wrote on Today, 21:32:

If none of the above helps, the first thing I'd propably do is scope the BIOS ROM select ... you should see lots of accesses (ie […]
Show full quote

If none of the above helps, the first thing I'd propably do is scope the BIOS ROM select ... you should see lots of accesses (ie: continuous if it never boots)
If it gets "some" selects then stops, that would be an indication of bad RAM (or possibly ROM).
This would at least tell you if the CPU and ROM selects are working - but... you need to have (or know someone with) a scope.

Some of the first things most laptop BIOSs do before even trying to boot are:
1) Test some RAM (look for lots of RAM accesses at reset)
2) Turn on backlight (does it come on?) - but it could be burned out or hardware reset state might come up on - so hard to tell unless you can scope a delay
3) Clear screen (can you see "junk" that clears on reset when you power-on (If no backlight then use flashlight as you mentioned))
4) Initialize keyboard and check for "override" keys (look at keyboard scan lines)
Again, to detect most of these you'd need a scope.

I'm afraid, I don't own a scope and I don't know anyone who does or would lend me one.

I really appreciate your suggestions, though.
It means a lot to me that you're putting in so much effort and trying to help me.
If anyone lives near me and would be willing to take a look, I'd be happy to send you my Libretto for further inspections.
However, since this is an international forum, I know my chances are rather slim (for the record: I'm based of germany).

But yeah, unfortunately a scope is a no can do for me at the moment.
I'm sorry.