Then it is possibly a coding error which reacts to that rc=4 incorrectly in EHP7. PSIGN=* should have no meaning as it can either be I or E.
'*' should be ignored.
If both trace results are the same, then the coding in the function module must have changed in how it reacts to it, or the calling BSP (looks like it is coming in with a logon ticket from a portal) has implications on the BSP side that dropdowns are not supported anymore in the way it has been coded. Eg. the search help has changed or the structure for the output has changed.
I tip on incompatibility with your BSP. SAP made a few changes to this FM recently...
Is there no BAPI for this?
What we do here is that we classify data objects and have a list of APIs for them. Then we scan our code to check that the call stack comes from the stables APIs and not some FMs which happen to be remote enabled. Same for correct use of methods which contain correct validations and checks - particularly if the data model changes.
You could try to open a customer message with SAP and tell them which FM you are calling, but 90% sure they will say it is not supported and 99,9% sure they will say that your BSP code is the problem.
Try it and let us know (if you did not do that already).
Cheers,
Julius