Been fighting this for a while. Resorted to AFP, but now sequoia warns that AFP is to be obsoleted in the next release. Cute.
Anyways, given is 5yr old 3tb ecrypted backup set, which started on El Capitan; Qnap 433; latest sequoia 15.6.1.
SMB time machining just didn’t work like for everyone else. And for me it was never a credential issue. It hangs for hours on ‘mountingdiskimage’, then craps out.
So, tried this How to disable SMB 1 or NetBIOS in macOS - Apple Support. Their ‘tee’ command line didn’t work for me (file got filled with 3 [defaults] lines instead of lines 2-3 being config keys). So I nano’ed it manually. After reboot it hung on ‘preparing backup’/‘mountingdiskimage’ for a while then… started backing up.
Phew looks like I got it to work finally.