The reference voltages are all okay. (+/-7V and there is one +10V test point, also okay).
I have not measured the current flowing in the ohms range yet. I wanted to concentrate on one problem at a time, but maybe I'm missing something. Can you elaborate?
I'm making (slow) progress in tracing the signal. There is one opamp I'll have another look at. It's showing strange voltages when I apply 4V DC at the inputs in the 10V range (all opamps before are fine). I'm also trying to figure out how a multi-slope integrating ADC works. There are 4 transistors and 4 voltages regulators all 3-pin SOT-343 or something similar with various voltages at the pins. The strange thing is, although there are various positive voltages which appear to make sense (like +20V, +10V) on the negative side it's all off and I measure strange values. I suspect something is wrong there, too.
What I am pretty sure of is that the FPGA is ok. There are active digital signals between it and its SRAM and at the optocoupler. Also, the integrator is showing activity when I manually trigger it or leave it free-running. I am 99% sure it's a problem in the analog part.
For now, I have a suspicious looking opamp to investigate, so I'm quite satisfied for the moment
For reference, if somebody ever has this thing on the table (note: based on observations/ measurements, 99% certain):
DCV range selection/ input divider (by 1, 10, 100, 1000): Z1/U37 (ceramic laser-trimmed resistor network/ multiplexer DG444)
1st opamp: U36 (LF353)
2nd opamp: U35 (AD8510) (either dead in my unit, or has a special purpose-- strange voltages, need to double-check)
integrator: C131/U49 (AD8510)
comparator: U50 (LT1394)
FPGA: ALTERA Cyclone I EP1C12 (FPGA used in the analog part of the PCB)
I'll be updating this with new findings. Hopefully, this is useful to somebody one day.