Summary: you have four USB scanners; plug them in, and SANE can access any one of them successfully, but then cannot access any of the three other scanners.
If you unplug one scanner, then plug it in again, can SANE then use that scanner? This action should re-create the device's data structures and thereby clear any status left by earlier activity. If the re-plugged scanner works, this suggests SANE changes the state of the scanners it did not use in some way that interferes with later use.
