Hello Daniel,
Even in SIMPLE recovery mode it can be the case that you have a long running transaction that does not allow the reuse of the log.
1) Try to increase the size of the T-Log
2) Check the SQL Server error logs for further details
3) If you notice the T-log growing too much, check out SAP Note 421644 on how to check for long running transactions.
4) Try to enable Auto grown for the T-log file to avoid log full situations.
Best regards,
Luis Darui