any option like allow legacy support on usb ?

This may sound strange, but please check the "case intrusion" jumper. If the board has it. Make sure it is closed (case intrusion should not be checked). I had a similar problem on Asus P5K deluxe - case intrusion was blocking the whole input system after boot. Also, check for any burned or damaged chips on the mainboard - especially near keyboard (PS/2) connectors.

Oh I have one of these boards, and it's also a problem with the TUV4X. Sometimes it will not detect USB devices at all. Usually resetting the the bios to default settings, save and quit fixes it. Sometimes (and this is no bs) you need to turn the machine off, and turn the power switch off and back on after some seconds on the power supply itself (let the LED on the board fade out). Then it will recognize USB devices again... I dunno if it's a BIOS problem or the VIA chipset.

I hope this helps.

No input devices are working. At all. That includes PS2, as I said before. The motherboard does not seem to recognize any input devices at all.

I cannot change the BIOS as the keyboard is not detected.

This may sound strange, but please check the "case intrusion" jumper. If the board has it. Make sure it is closed (case intrusion should not be checked). I had a similar problem on Asus P5K deluxe - case intrusion was blocking the whole input system after boot. Also, check for any burned or damaged chips on the mainboard - especially near keyboard (PS/2) connectors.

I tried that, to no avail. I also checked around the board and everything seems to be physically in good condition.

any option like allow legacy support on usb ?

I cannot enter the BIOS to check it and my board does not have a jumper for it. But the default settings of it are on "Auto", so I don't think that would be an issue.

Oh I have one of these boards, and it's also a problem with the TUV4X. Sometimes it will not detect USB devices at all. Usually resetting the the bios to default settings, save and quit fixes it. Sometimes (and this is no bs) you need to turn the machine off, and turn the power switch off and back on after some seconds on the power supply itself (let the LED on the board fade out). Then it will recognize USB devices again... I dunno if it's a BIOS problem or the VIA chipset.

My problem is not limited to USB, but I have tried it anyways. It didn't work out, unfortunately. Neither PS2 nor USB devices were recognized after that procedure.

This is a longshot, but make sure the CMOS battery is good. Some boards start to have weird problems when the battery gets weak.
Other than that, I'm wondering if the 5V standby rail is having an issue, either on the board or PSU side is equally likely. This is also a longshot given that the PSU worked in another machine.
If you are able, it might be worth checking all the various voltages on the board with a meter.

This is a longshot, but make sure the CMOS battery is good. Some boards start to have weird problems when the battery gets weak.
Other than that, I'm wondering if the 5V standby rail is having an issue, either on the board or PSU side is equally likely. This is also a longshot given that the PSU worked in another machine.
If you are able, it might be worth checking all the various voltages on the board with a meter.

The PSU is fine, I've even switched it in hopes that that might be the issue. Apparently not. I switched the CMOS battery to no avail, as well. Which points in the board I should check with my volt meter? I checked the PSU and all the voltages are good, but I'd have no clue there to test the boltages on the motherboard.

The most straightforward would be to measure the inputs at the ATX power connector. It's easier to do that from underneath the board, but it might also be possible to backprobe the connector. To do that you'd slide the probe into the top of the connector, far enough to make contact with the uninsulated end of the wire.
If you put the probe under the board or anywhere where you're worried about shorting anything, it's a good idea to wrap some masking tape around it so only the very tip is exposed.
You can put the ground probe somewhere safe on the frame of the power supply or any grounded part of the chassis.
The point is just this is a way to get operating voltage readings without being able to get into the BIOS health monitor.
Specs allow voltage within +/- 5% for each rail I believe. If it's anywhere close to -5% then it's questionable though, especially when ripple is taken into account (which the meter can't see).

If you want to try this, the ATX pinout is attached. Most of the pins of interest are in the corners, but 5Vsb isn't. I think it's common for 5Vsb to be involved in powering the I/O.

Beyond input voltage, I suppose the next step would be to check voltage at the pins on the I/O controller chip, to see if it's getting good power or the chip itself is suspect. That's getting beyond anything I've ever messed with though. And it's hard to make sense of why it would work once every boot..

My last multimeter was having some issues, so I had to buy another one, sorry for the delay.

I measured the inputs and they are... Perfect. The variation from the default is pretty small, around +3% average. Even the I/O chip seems to be getting power, as well. This problem is getting mighty disturbing.

The board also may have bad capacitors, this one was manufactured in that era.

Check if the 5V are available on the USB and PS/2 connectors. Sometimes there are fuses.

Edit: Ok, they still get power as written in your first post, forget this question...