I guess that bbswitch package, as well as what is done in nvidia packages, should remove old configure files like
/var/lib/dkms/bbswitch/0.8-5/
during postintall phase because it prevents dkms install to work properly when new kernels are installed.
If I am right, this is an old problem with the bbswitch packages in OMV.
During installation of the new kernel I got,
Error! Could not locate dkms.conf file.
File: /var/lib/dkms/bbswitch/0.8-5/source/dkms.conf does not exist.
ERROR: 'script' failed for
error: %posttrans(kernel-release-desktop-4.15.7-1omv-1-1.x86_64) scriptlet failed, exit status 4
The only option now is to manually run dkms install for bbswitch and also for nvidia drivers because dkms is stopped due to errors with bbswitch.