Lately I had to realise that the OwnCloud support for SQLite is literally only for testing, even if you have a single user, despite what I have read out of the documentation (which states you could use SQLite for few user sites).
I had several issues with my calendars as well as my contacts (failed to sync, failed to import, etc.), which all vanished when I migrated/converted the database to MySQL. Luckily, OwnCloud has support for exactly that use case. Just invoke the following (given you have setup php correctly).
I just wanted to delete all tags set in my Applications folder on OS X and had to look up how to accomplish that in the Terminal since it a) is quite cumbersome to do it in Finder and b) it did not quite work somehow.