Manual check process fails randomly, but runs successfully with DEBUG on?

We have a client on the AS/400 platform that upgraded from 9.0 to 9.1. They have been having issues since Go Live running A/P Manual Checks (auto checks run fine).
Manual Checks fail in Error, usually with R04570M ending in Error Status, and sometimes we additionally get R04572 in Error status. On a good run, R04570M, R04571, R04572 would all have run successfully. The error log included is not much help (see below). The odd things is when we turn on DEBUG logging, we cannot get the job to fail. We have Deleted SQL packages, cleared spec caches, built and deployed a full package, fixed an issue with OS/400 Journaling and did an UNDO on all Payment Group and cleared F04571, F04572, F04573. We continue to get the errors (sometimes random manual checks process successfully but never more than 2 in a row. Yet with debugging turned on, we cannot get it to fail! Anyone have any clues, thoughts?
11467/882 WRK:Starting jdeCallObject Tue Sep 29 10:02:53.476400 jdb_drvm.c921
JDB9900169 - Failed to initialize db request
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PID/PSThread_ID: 111467/882
System Thread ID: 882
Thread Name:
CALLSTACK:
Level Program Library Module Statement Procedure
0 JDELIB E910SYS JDEDE BUG 0000000035 allocThreadStack
1 JDELIB E910SYS JDEDE BUG 0000000092 allocAllThreadStack
2 JDELIB E910SYS JDECALLSTK 0000000004 logCallStackOnAS400
3 JDELIB E910SYS JDECALLSTK 0000000004 jdeLogCallStack
4 JDENET E910SYS NETSIG 0000000002 krnlCatchall
5 JDEKRNL E910SYS JDB_DRVM JDB_DBInitRequest
6 JDEKRNL E910SYS JTP_CM 0000000105 CM_FindDriverHandle
7 JDEKRNL E910SYS JTP_TM 0000000034 TM_DBPerformRequest
8 JDEKRNL E910SYS JDB_EXEF 0000000220 SelectKeyed
9 JDEKRNL E910SYS JDB_EXEF 0000000003 JDB_SelectKeyed
10 JDEKRNL E910SYS K2SECINIT 0000000111 InitializeSection
11 JDEKRNL E910SYS K2SECINIT 0000000003 K2ProcessInitSection

Joe,

It might be an issue of debugger slowing down the process enough that an asynch event has enough time to finish.
When running on the server that same event might not complete by the time the next event is executed.
Check to see whether the called UBE's are noted as asynch w/in the ER.

Hope things are well.
Sorry to hear about M Simon's passing.

Gene

Thanks Gene, hope you are well.

Odd thing is this is a completely vanilla process. P0413M launches the UBE's for the manual check process (R04570M, R04571, R04572). The objects have always been synchronous and worked in their prior release (and work for all our other clients). It appears to all run in a single job/process on the AS/400, synchronously. The randomness of the issue (when DEBUG is off) is frustrating, and certainly perplexing that once DEBUG is turned on, it seems to run fine repeatedly.

As Gene says, from experience, this is because muti-thread is turned off in debug mode, so look for mods that have an async business function or UBE being the source of the problem.