Time Machine stopped backing up

Part 2: I just found that if I click on "Backup delayed" in the Time Machine drop-down menu at the top of the screen, a window appears stating:

"Time Machine cannot back up because some iCloud Drive files have not finished syncing. Syncing happens when your Mac is turned on and connected to the internet. Syncing may take a few days, but will happen more quickly while your Mac is connected to a power source".

iCloud was full yesterday, so I deleted loads of unimportant photos and other things such as Whatsapp voice messages which I THINK might be synced to iCloud, if I recall correctly).

Never had iCloud interfere with Time Machine back-ups before....

Hi D.I. Johnson, thanks for responding. I have actually had iCloud full before, but not this problem with backing up. It's frustrating – if this iCloud issue really is the problem – that the notifications / pop-up windows etc. are not more specific in the first instance (i.e. before clicking on various things looking for the problem over the course of an hour before seeing the iCloud info – "backup delayed" was about the last thing I clicked on. It looked like just a message, and I didn't realise I could click on it for more info). My need to back-up work with Time Machine is far more important than my need to use iCloud (only relatively trivial photos, voice messages etc.). "Syncing may take a few days"? I hope not. Will report back on how it goes. Thanks.

Update complete. It only took 3 days for iCloud to sort itself out, so I could back up with Time Machine again. iCloud had been close to full for a few weeks. I kept deleting a few unimportant photos here and there, and meant to find time to delete a lot more. Then on Friday, I made a short video that pushed iCloud to "full" and then I deleted a bunch of photos and videos. I'm very surprised that hitting the limit in iCloud (for about one hour only) made it impossible to back up with Time Machine for 3 days!