Decrypt problem after sync error

I have been using Joplin for a couple of month now and it is exactly what I’ve been looking for for ages. Just great! Today I faced an error which resulted in unretrievable data.

macOS Catalina (1.0.199, re-installed today due to my problem, might have been more recent version before uninstall)
iOS (10.0.47, however problem started to occur with earlier versions already)

I encrypt all entries and use WebDAV for syncing which used to work flawlessly, until some days ago when I got this sync error on all devices (sync status was still “successful”, though):

Last error: Error: Sync version of the target (11) does not match sync version supported by client (1). Please upgrade your client.

While everything still seemed to worked, I realised today that it had stopped syncing properly a week or two ago. When manually comparing, I found 8 unsynced entries on macOS client.

Now, I wanted to set up everything again:
What I did: I copied and backed-up all encrypted entries and changed WebDAV folder
What it caused: After changing WebDAV folder, on macOS and iOS the error message disappeared
What I intended to do: Recover those 8 highly important entries that were only on macOS client before
What happened: Re-sync with original folder does not work, import of encrypted md-files gets me an import of the encrypted text, sync with duplicate folder does not work as well – just nothing would happen
What I need: Support on how to retrieve the missing 8 entries