Remove unknown deletes btrfs snapshot but fails - read only filesystem

Trying to remove unknown backups from the filesystem, the utility tries to delete a subvolume snapshot. It hangs momentarily, and then continues one, failing to delete anything further. I unmount the parent filesystem (/backups - btrfs), then remount it and try again. The utility tests for btrfs, succeeds, and moves on to cleaning out unknown backups, and fails again trying to remove the same subvolume snapshot. I suspect filesystem corruption, but can’t manually remove the subvolume. Looking for guidance.

I suppose you already checked that the mount is read/write and tried to set the snapshot with the btrfs util to rw (read/write)?

Yep. Weird: it says I don’t have permission. I suspect the filesystem has been corrupted.

Uhh that can get messy.
Except in 2018 I never had a corrupted BTRFS. And than it was because of quotas. Couldn’t salvage the data with the know possibilities at that time.

A scrub can give you evtl some hints what the problem is.
But otherwise to solve your issue you evtl. have to look to BTRFS specific forums/mailing lists for help.