Ran into this wonderful WordPress “feature” with MicroKid Related Posts. I would get the error when saving “Related Posts” admin page options. Search hits like this didn’t apply. Browsing results made it seem like this error could’ve been caused by about a thousand different things, including your dog farting too loud.
Problem: plugin directory name “microkid-related-posts.svn”, since I check them out directly and like to know the scm type used
Solution: remove “.svn” from directory name
Should anyone know why that helps, I’d welcome some hints.