Turn on the "tRFC (In Background Task): Block Sending" flag in the debugger settings
Go to SM58, select the tRFC
EDIT -> debug LUW
try SM66 if you have several application servers, OR you used WAIT UP TO n SECONDS. So, I can't tell you if it will show up in SM66, especially if the issue is the other option.
The debug option is available in SM66 from Basis 7.40. If you have a previous release, then you need to check on which application server is running your endless loop via SM66, then you switch to this server via SM51, then you start SM50.