possibly cracked solder joints under the ADB jack. Try re-flowing the solder. Check the fuse at F2, it may need to be replaced. If both check out, try re-flowing the solder to the transceiver chip leads at UJ6. If the chip is bad, you'll have a problem as there are no replacements for it and are difficult to remove from junk boards without damaging them.

It could also be a software problem. I've seen that behavior particularly in a PowerBook. But usually a restart would fix it. Did you try booting from the ROM System RAM Disk? If it still won't work with known good mice, it's definitely a hardware problem.