Can't back up to Time Machine - it says alchemybacklightbounce can't back up to my external drive

Hi. My Time Machine has suddenly stopped backing up to my external hard drive. There is plenty of free room on the hard drive and it's been working fine for the last couple of years. The error message I keep getting says: “/Library/Apple/System/Library/Receipts/com.apple.pkg.MAContent10_AssetPack_0837_AlchemyBacklightBounce.plist” could not be backed up.

I've tried to exclude this file from backing up but it does not come up in any searches. I've restarted the MacBook in Safe Mode. It's still not working. Any suggestions would be much appreciated.