OS error 13 when running mariabackup as non-mysql user
I was able to use the strace command to figure out what was going on. There were two problems. The first is that there was no group write permission on the output directory, so it failed to create the xtrabackup_backupfiles directory. The second is that there were two database directories in /var/lib/mysql that lacked group r-x permission. By fixing these two issues, I was able to get it to work.

Still, it would be nice if the error message actually indicated what specifically it didn't have permission on, rather than simply reporting that it lacked permission for something.