The “database is locked” error indicates a problem with your sqlite database. This can happen if the database is left in an inconsistent state after a crash, or if there are problems with the disk.
One thing you can try is to dump the data from your existing db file into a new one, then swap it out. From inside your Grafana data directory (after shutting down Grafana):
sqlite3 grafana.db '.clone grafana-new.db'
mv grafana.db grafana-old.db
mv grafana-new.db grafana.db