Obnam had a bug in which "obnam forget" would remove a chunk before removing references to the chunk. If the "obnam forget" was interrupted, the repository would have references to chunks that had already been removed. This would cause a later run of "obnam forget" to try to remove a chunk that no longer exists, and that would cause a crash.
The crashing was fixed in Obnam version 1.13. As of that version, "obnam forget" will ignore that a chunk is missing if it's removing that chunk anyway.
There remains a problem, unfixed as of Obnam 1.16, where the chunk still exists in lookup indexes, and "obnam backup" may try to use the chunk.