Restore Backup says file invalid

I have 2 MacBooks, one which I use for travelling. I work on the bigger  MacBook Pro 16”  at home. About to go away I made a backup, saved to a Dropbox account, from which I restore to the “travelling” MacBook Air 13 computer when I go away.

But now it’s refusing to restore from the backup. It tells me it’s an “invalid file format”. It restores fine to the machine on which the backup was created. And I get the same message when trying to restore from backups that have previously been successful  Both are running Basecamp 4.8.12 and say their up to date. Both are intel based Macs running Most up to date version of Monterey.

Any ideas gratefully received

john