The problem is not in fact caused by BAPI_TRANSACTION_COMMIT. What is happening is that when the commit happens, the form UPDATE_FROM_INT in LMCB1F10 is called. This is because mcinf_update_all in LMCB1F10 has this piece of code:
PERFORM update_from_int ON COMMIT.
Therefore the problem lies firmly within the Inventory Controlling module, specifically in the updating of Inventory Controlling Statistics.
Looking at the code within the form issuing the error, it looks like a program isn't registered in some configuration table. Try re-running your test and find out what the value of prog_tab-fprof is a the place the error in thrown. I'd bet that it is a Z program - perhaps your "Z" Bapi.
No, you've hinted that you've copied a standard BAPI into the Z name space for your own purposes. Not only does this cause problems such as you are experiencing, it is also a profoundly bad thing to do. The reasons it is bad are:
1. Patches and fixes to the original BAPI won't get applied. This can lead to your Z-bapi becoming syntactically incorrect, or even having (over time) incorrect logic, leading to corruption of your data.
2. It is exactly the same as writing your own code to update standard SAP tables. If that happens, then the terms of support with SAP will almost certainly have been broken, and your client/employer will have to pay $$$$$$$ to fix it.
I strongly recommend you find another route to address the business requirement. Modifying the standard is a safer approach than cloning a standard program. Perhaps if you shared why you've made a copy, suggestions could be made as to safer ways to achieve the requirement.