I switched from Whatsapp to Signal (in terms of many communications are now on signal, but still some are left on Whatsapp) and afterwards i moved to a new smartphone.
But while doing the restore procedure for the backup (take a look here) i got this error:
Data backup of newer signal versions cannot be imported
or in german
Datensicherungen neuerer Signal-Versionen können nicht importiert werden
I investigated the version numbers on android playstore and both were 5.2.3.
On my new smartphone the android os was not on the latest release (on this new smartphone there was still some outstanding os versions to install).
But nothing did the job - i asked signal support, so let's see, what they are telling me...
EDIT: Even deinstalling signal on my old smartphone and reinstalling signal showed this error message...