Thanks everyone! I finally got it to work!

Seems like the problem was with the termination.

The drive inside the case is an IBM WDS-L80 (probably) with Apple ROMs. Since it came in the LC II without any jumpers installed, I thought it was terminated by default.

Maybe, it wasn't so.

Installing a standard 50pin external terminator on the 2nd SCSI port of the drive enclosure made it work.

So it's double-terminated now? Is this OK?