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).