Comment # 6 on bug 982329 from Manfred Hollstein

FWIW2: I just tried to reject /dev/loop devices from lvm's filter by adding

  "r|/dev/loop.*|", 

to devices/filter in /etc/lvm/lvm.conf, set max_loop to 64, rebuilt everything
required, rebooted and failed again. Even if the /dev/loop devices are
explicitly excluded in lvm.conf, they appear to be scanned when the system is
booted, and 64 non-existing loop devices appear to be too many to allow the
dm/udev logic in the latest lvm2 package to succeed.

I'm now back to max_loop set to 32 :(