https://code.launchpad.net/~vcs-imports/kdesupport/trunk and other branches fail to import.
The root cause here appears to be that layout.get_project_prefixes returns a list including 'tags/projectname', which is a path which does not exist.
During find_tags_between, RevisionMetadataProvider.iter_all_changes is called. This starts up a RevisionMetadataBrowser, feeding it the prefixes from layout.get_project_prefixes. These prefixes then get handed on to CachingLogWalker.iter_changes, and thence logwalker.py iter_prefixes_changes. That function incorporates the fundamental assumption that any prefix it is passed exists in the from_revnum that it is passed. Thus, we get an assertion fail.
Wow, that is some awesome debugging.
I've got a basic fix for this that I'm testing at the moment.