Time Machine is stuck on Preparing Backup

I’m trying to find a solution to Time Machine stuck on Preparing Backup, and hoping someone may be able to help. TM has been super-reliable up to this point, but now it doesn’t progress beyond this stage.

Tried stopping current backup, deleting the ‘in progress’ file, and starting again - without success.

Ran Disk Utility First Aid on the external drive, and all showed up OK. Ran TM again, and it did a full backup (although it took 10 hours). All worked OK for three days. Now it is stuck on Preparing Backup again, and when entering TM, previous backups only go back three days, with nothing before that. All previous backups are still there in the external drive, but don’t show up in TM.

This is on an iMac, using OS Catalina.

Any advice would be appreciated.