1. rebuilding the database (forced rebuild)
2. restoring the database from a backup
1. has been solved by Boris with the update 14426 for MB Pro from 28-Jun-2021 - Thanks for the fast action !!

2. is still an open issue (which will earlier or later hit all MB Pro users - see below)
Background
My MB database is bigger than 500 MB meanwhile and the internal memory, where the database "sits" is 960 MB only in a MB Pro.
The rebuild script used the internal storage only and during the rebuild, the existing database is exported to a compressed file dbase.sql.gz and and new database dbase.new is built. Of course 2 * 500+ MB plus about 200 MB compressed export won't fit into 0.9 GB storage.
Boris corrected the script and offered the possibility to choose an external USB for the rebuild process in the system/administration page.
I tested it - and it worked (took 7 hours to export the 600 MB database and 6 hours to rebuild it, but it worked).
Now, I tested the restore and ran into the same issue - not enough storage available.
Obviously the database backup is copied into the internal storage where already (still) the old database resides. 600 + 600 > 960.

The only way to make this work for me was to SSH into the MB, delete the dbase* files from the /tmp/mnt directory where they reside, reboot and then the backup would run.
I think a new approach is necessary as not every user wants to/can/knows how to work with SSH and possibly mess with the file system.
At least an option for databases whose size * 2 is bigger than the available internal storage.
A possibility I could think of would be an option to use the USB stick not only to copy the backup from but also for the whole process as a temporary or permanent swap file
e.g. copy the old database files (dbase*) onto the USB stick, then delete the dbase* files in the internal storage, copy the backup into the internal storage and do the necessary work (rename, restart, re-create the RAM-DB ....)
The old database could be kept on the USB drive/stick for those (experts) who want to do some research with them.
When the size of the backup file plus the old dbase files surpasses the available internal storage and a restore is initiated, an error message should be posted (log file) telling the user e.g. "not enough internal memory for database restore, please select USB swap option [to be created] and make sure your external USB storage has double the size of your backup file free storage available" - something like this
Any other working solution is of course also welcome.