after some trouble with CD/DVD I nailed down the problem to cdfs.sys. This was not started (and CD with IS9660/Joliet were not readable). I tried to manually start it, but failed. Then I change start-code from 4 (never start) to 2 (start automatically) and rebooted. But still got the same error as with starting manually:
"Windows cannot verify the digital signature for this file. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source."
I Treid sfc /SCANNOW, but did not get any reports regarding cdfs.sys. I also tried fc to compare the existing cdfs.sys with a version from a second PC -> identical. I also ran sigverify, but this also did not report any problems with cdfs.sys
I am out of options now (easy stuff, like driver uninstall etc already tried). What else could I try?