The four most problematic servers were hanging nearly every day. They each have a dedicated hard drive and are running 4.19.0-0.bpo.2-amd64. I installed btrfs-progs 4.20.1 on each and recreated the file system on the hard drive. There have been no issues since then, but it is too early to tell. There are hardly any references or files on the drives. It might take up to a month to know if there is an issue.
--
Russell Mosemann