Seems the database was locked (for whatever reason) during one of the migration steps. I have lots of these entries in my log file;
"message":"One event could not be migrated. (id: 1069, calendarid: ): {\"Exception\":\"Doctrine\\\\DBAL\\\\Exception\\\\DriverException\",\"Message\":\"An exception occurred while executing 'INSERT INTO \\\"oc_calendarobjects\\\" (\\\"calendarid\\\", \\\"uri\\\", \\\"calendardata\\\", \\\.... .......database is locked\",\"Code\":0,\"Trace\":\"#0 \\\/usr\\\/local\\\/lib\\\/owncloud\\\/3rdparty\\\/doctrine\\\/dbal\\\/lib\\\/Doctrine\\\/DBAL\\\/DBALException.php(116): Doctrine\\\\DBAL\\\\Driver\\\\AbstractSQLiteDriver->convertException('An exception oc...', Object(Doctrine\\\\DBAL\\\\Driver\\\\PDOException))\\n#1 \\\/usr\\\/local\\\/lib\\\/owncloud\\\/3rdparty\\\/doctrine\\\/dbal\\\/lib\\\/Doctrine\\\/DBAL\\\/Connection.php(996): Doctrine\\\\DBAL\\\\DBALException::driverExceptionDuringQuery(Object(Doctrine\\\\DBAL\\\\Driver\\\\PDOSqlite\\\\Driver), Object(Doctrine\\\\DBAL\\\\Driver\\\\PDOException), 'INSERT INTO \\\"oc...', Array)\\n#2 \\\/usr\\\/local\\\/lib\\\/owncloud\\\/lib\\\/private\\\/db\\\/connection.php(205): Doctrine\\\\DBAL\\\\Connection->executeUpdate('I