Thank you for your post. Diagnostics indicate that the Nucleus was indexing a subdirectory location that might have been undergoing backup by a third-party service or was otherwise unavailable. The Nucleus couldn’t gather file information - and at points couldn’t access - the files associated with local library tracks.
If a restart of RoonServer hasn’t resolved this issue, then verify that the Nucleus has reliable network access to the storage location and share a screenshot of the Nucleus Web Administration Page and Watched Folders path in Roon here.