Not sure what it is.... what I can see;

(green ellipses).... obviously yet another victim of varta type leakage corrosion damage .... it's kind of odd there appears to be evidence of this damage in various places. The alkaline fluid would've creeped under the power connector -- there's no evidence of that connector being lifted to check underneath. If this is the board with the bad serial port, that mess (red ellipse/boxes) are more than likely got something to do with it (rs232 RX/TX pairs)....the mess is a prior repair attempt of the serial lines ( why the need for repair in the first place?) .... datasheets for wd76c30 in the wild...not too much guess work here.

....what does the back (solder) side of this board look like?

Thank you for your response.
Yes, unfortunately, the battery has leaked significantly. I cleaned the board with a fiber pen and R-11 contact cleaner. I will provide a link to a picture of the backside of the PCB.
https://1drv.ms/i/s!Au6NnqmpfWWHidxVVGGk0FweESqLXQ?e=Zpa4WW

....yeah, {groan}... pretty much as I expected -- corrosive liquid has run to underside of board and along periphery of PCB, and in places bubbled up through various vias...

....everything in the green box needs clean/inspect/rework. Serial port lines are on the left (white markings), that go to topside thru those vias ...

...and come out there obscured by ram socket, and the vias on this side appear corroded... datalines leading down to max1488/1489 rs232 ICs...

Board needs to be washed in citric acid, rinsed in clean water, steeped in IPA, compressed air dried and then damaged areas reworked accordingly =)

What's the other board like?

On the second motherboard, fortunately, the battery was removed before it started leaking, and there are no damages. However, unfortunately, upon booting, a KB/interface error message does appear. Initially, pressing F1 gets you into the BIOS, but you can't navigate further from there. When pressing keys, three LEDs on the keyboard start flashing (I have already replaced the keyboard controller chip on the motherboard, but the issue persists). I will include links to pictures of the motherboard:
https://1drv.ms/i/s!Au6NnqmpfWWHidxwyfvr_ZpzYQ5y3w?e=2H486q
https://1drv.ms/i/s!Au6NnqmpfWWHidxv6uO7iKK2gbachg?e=hNU2kn

if it takes F1 key and then when pressing keys led flash then keyboard communication works just fine, problem is somewhere else, maybe some magic software lock

I'm wondering is this is a diskless client -- did either machine come with HDD? (or evidence of one being fitted?)

Obviously, the second, non-damaged board is the one to focus one...

rasz_pl wrote on 2023-10-05, 14:53:

if it takes F1 key and then when pressing keys led flash then keyboard communication works just fine, problem is somewhere else, maybe some magic software lock

You'd think so.... best I can gather, these had a 90degree riser in board slot center, which carried a NIC ~ I wonder if it's trying to netboot? I'd be suspicious that switch block next to KB connector holds some sway here.