How to restore a Backup

I’m back from vacation and try again to get my data. Joplin would be great, but if my data lost so easy and there is no possibility to get back it from my backup I think its not useable. This can happen all the time…

So - does anyone have a solution?

What I tried today is to import my backup dir via Menue “file - import - md (directory)”. This will import my backuped files. But only in one notebook and with the filename e.g. ffae3d15d81f4628b1c7658fc1db17d9 as a title. So this is not usable.
I also tried to make a new, different location on NAS with the backuped files and use this as sync location. But this also didn’t work. The files there are ignored.

So one again, how can I get back my backup of original stored files from joplin sync directory ?
I can provide log-files (eMail, upload), if this will help.