Hi @jukkaforss, and welcome!
Thanks a lot for reporting this issue… The Time Machine compatibility is always tricky to test for us due to limited and difficult access to Apple hardware; reports such as yours are thus extremely helpful to keep up-to-date.
A quick question to rule out a possible issue related to macOS migration. Was this Time Machine backup setup and working well in a previous macOS version or this on a brand new machine and thus a first time setup of that Time Machine backup?
From that error message, it seems Apple might have updated the requirements for Samba support so we’ll need to see what is needed and once that is figured out we’ll do our best to implement it. Maybe someone on the forum has come across that and know what needs to be updated? That would save a lot of time and be greatly appreciated!
Thanks again for the report, hopefully the move that seems to have happened with regards to Time Machine requirements is well documented as that will make everything much faster.