Time Machine 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.

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

Also tried running Disk Utility First Aid on the external drive, and all showed up OK.

Any advice would be appreciated.