Yes there is an backup. But I now decided to give up on this one. I setup a new and fresh 4.1 system and spent the last hours to do a manual configuration of all my devices from scratch 
It seems that I am already done 90% so there I don’t want to mess this new installation with backupfiles which (as you pointed out) would only work in steps to 3.4. and then up … And then still you cannot be sure if you keep everything.
The last days were quite frustrating as I am almost 100% sure that by updting the firmware of my Tapos those devices cannot be handled right now by OH anymore. (I already posted this separate issue: P110 via TapoControl provides unknown API error (-1)
Then I made the mistake to think: Well it’s time to update anyways. And this crashed my whole system.
The learning from that is: Always try to keep your systems uptodate. From what I understand, that means, you need to constantly check whethter there are new openhab images releases, then kill your old container and install a new one and mount the old directories (conf/userdata/addons) right?
Is there a chance to get automatically notified whether there is a new image available?
And even more interesting: Is there a way to automate all this? Although I think that this is something which does not any longer belong to this thread (as it’ll be handled in general by docker? Right)
Thanks anyway for your help.