I have moved quite a bit forward now. I have ascertained that the problem IS connected with my Backup folder. It appears that RB fails to store a backup after the first backup file is stored in the folder. My songs are on a different drive to the RealBand folder so the backups are stored at a folder on the root of the song drive.

When I looked in this folder there were clusters of 5 backups, all nicely sequentially numbered. My preferences allows for 5 backups which I assume is the default value. So the system has been working. Listing these backups by date descending I noticed that the only clusters of sequential files were those just prior to my installation of the 2021 version of BB/RB.

This concurs with my experience of the particular issue that I posted about in the first instance. So, what is happening as far as I can see is this:

I save the SEQ file for the first time generated from my SGU. A backup is stored in the backup folder and original SEQ file can now be replaced by the TMP that represents the current saved SEQ.

When I save again, even without modifications, RB tries to store the SEQ as the next sequential backup but is failing to do that. As a result the original SEQ file is not deleted and the TMP file remains in the songs folder. As a workaround I can delete the SEQ and simply rename the extension of the TMP file to SEQ.

But the workaround is quite inconvenient so I went to preferences and reduced the maximum number of backups to just 1. RB is quite happy now because it always moves the last backup to the Recycle bin and I've always got a current backup in the backup folder. I can live with that, but I just wonder that I am the only one suffering from this problem.

It's definitely not a permissions problem because I did the same thing on my laptop using a FAT32 USB as a song and backup folder with exactly the same results. In any case the backup system was working normally prior to the installation of 2021.