OK, it seems that the field in question is NOT the "Source Code" field, but the "Source No." field, which is an internally used field and is neither editable nor visible in the Item Journal.
I've ran the same procedure as you have mentioned in your first, and I noticed the Source No. field was left blank by the system. However, I was able to post it just fine, and the system didn't give me any error messages neither in the "Get Standard Journals" function, or when posting. Are you sure that this feature isn't customized in your system?
Also, regarding the debugger, please ensure that "Breakpoint on Triggers" does NOT have a checkmark against it. Then, run the process again. This time you should get an error message and a yellow arrow in the code that shows were execution stopped. That's the point of code I want to look at.
Please advise.