how to debug a queue

Let's say you want to debug a queue, source R3, target CRM, when you run the adapter/business object from CRM you actually start an outbound queue (triggering queue), to have this queue stopped in SMQ1 in order to debug it just type /h before to press F8 in R3AS, and change debug settings according to this picture.
You will then find the queue stopped and you will be able to debug it from SMQ1.

Exception condition "GUID_FOR_LOGSYS_CHANGED" raised.

Data in tables CRMMLSGUID and CRMPRLS are conflictual.
Did anyone performed a restore in the source R3 system? A dbcopy? Whatever... You should follow the checklist to connect a CRM system with a backend R3 and fix it. But downloaded data in CRM system is much probably compromised.

Delete old bdoc log in smw01

Report SMO6_REORG is what you are looking for, it generally nuke a lot of old data, just give it the number of days of log data you want to keep and it's done.
To be more specific check the source code it does call different report to clean up specific data, e.g. RSMWM_BSTAT_REORG to clean up Bdoc statistics.

It's good practice to schedule SMO6_REORG so it's executed every day deleting old (1 week?) data.