My iPhone stopped backing up

Plenty of space left where?

Go to Settings > Your Name > Apple ID, iCloud, iTunes & App Store

Tap iCloud

How much space if available there?

See if anything in this link helps you --> If you can't back up to iCloud or restore from iCloud backup failed - Apple Support

Backup where? iCloud or on a computer? Which error message do your see when you attempt to backup your iPhone?

Axel F.

You didn't answer my question about how much space you have available in iCloud to hold backups.

110 Gb iCloud space, the back up takes only 32Gb

Iphone 11 pro max