Prevent the tmscsim driver from resetting the SCSI bus on startup. Upgrade your driver to 2.0d2 or later. Look at the main DC390 page and download the sources. Replace the 600 by 3000 or whatever value necessary for your device. There some reports with scanners causing problems when scanning with SANE, but I didn't manage to solve these yet.
More reports are welcome.
Basically, the scanner scans the image, but upon completion, there is a hang. Andreas Rick tracked this bug down to a Selection Timeout which resulted in the command being requeued again. As no further commands were sent to the scanner, this command was sitting in its queue forever. In 2.0d18 the Selection Timeout is returned to the mid-layer and this seems to solve the problem. Looks like a buggy SCAM implementation in the JVC firmware to me.