I tested them all (tantalums), no shortage. I tried with another CPU as well, no change. I'm wondering if a track on the board could have been broken when I removed/put back some cards due to the mechanical constraints, but visually I can't see any cut. I found that page https://dosdays.co.uk/topics/bios_ami.php with BIOS code, my bios looks like the AMI Color one.

@rasz_pl I did the tests you suggested with red wire to ISA B3 B29 D16, same for yellow wire and B9, on each isa slots, got [0-1] ohm, that seems correct. I tested all ISA termination resistors as well, seems okay, all around 4.7kohms. Tomorrow I'll try with removing all the cache chips, and check for chipset pins. Thanks for your advices