When Comet Cache Feed Caching is enabled (Comet Cache → Plugin Options → RSS, RDF, and Atom Feeds), a fatal error is produced whenever the cache is cleared.
For technical details, see this GitHub issue: https://github.com/websharks/comet-cache/issues/739
This is a high-priority issue and I’ll be pushing a maintenance release shortly to fix this issue. In the meantime, I recommend disabling Comet Cache Feed Caching (Comet Cache → Plugin Options → RSS, RDF, and Atom Feeds) to work around this issue.