I had problems with rdiff-backup failing, throwing backtrace ending in
"assert mirror_rp.isdir() and inc_rpath.isdir()"
Instead of handling this as an exception properly i was sent on a long
goose chase until finding out the backup was damaged: in this case the
increments directory was gone.
So *please* make it at least print the names of the directories it's
checking for. Preferably even try to fix situation or at least provider