Thomas -
a fix for the CUPS issue reported in this thread was added to CU 175 (testing). Can you give it a try and lets us know if it solves your issue?
https://blog.ipfire.org/post/ipfire-2-27-core-update-175-is-available-for-testing
https://bugzilla.ipfire.org/show_bug.cgi?id=12924#c36
Best regards,
Jon