Hi Ruben,
Using the recovery mode "SIMPLE" is suggested only for a few situations (during upgrade for example). But it doesn't mean that your Transaction Log cannot get full. If you have a big transaction (larger than your Transaction Log) it may get full.
Please check the following link which explains how to use DBCC OPENTRAN to check which transactions are active and not allowing the transaction log to be truncated or recycled:
You also might want to extend your current transaction log (autogrow is a good choice or set up the size according to the transaction/activity you're performing).
remark> 1GB for Transaction Log in SAP system is a very small log size. Try increasing it to 5GB for example. And for Production, activate FULL RECOVERY and start the Transaction log backups as soon as possible!
Regards,
Luis Darui