Hi,

I had a similar problem while installing excalibur (with another scanner):
But at my previous daedalus installation my scanner was working as user too.

After a long time of investigation, I found the problem at hwdb.bin. Therefore I copied /usr/lib/udev/hwdb.bin from my daedalus installation and I could use my scanner as user again.

I assume the cause of the problem is:
This file is generated during the installation of eudev and if at this time libsane1 is not yet installed, that content is missed.

I have not checked this, because I could use my old hwdb.bin, but I guess with the proper order of installation - maybe reinstalling eudev, there would be no user problem with the scanner.

Maybe it's also OK to generate this file later (even as user)
  udevadm --debug hwdb -u -o ./hwdb.bin
and copy it as root (after making a backup) to /usr/lib/udev/hwdb.bin.

But of course it could also be that your problem is completely different ..