Our site had a similar issue back in June 2020 which BMC Support resolved. If you haven't already I would suggest you open a case with them and refer them to our case (now closed) 00892048. This message can result for a number of different reasons. Ours was due to:
After consulting with R&D, this is the feedback provided:
The web services response contained a lot of special characters that would have required the Control Module (CM) to handle it in binary mode, and the CM (before the fix) was handling it in non-binary mode.
The fix provided was specifically for a different scenario such as attachments in web services response.
However, because behind-the-scene, the fix handles the response in binary mode, it turned out to be a relevant fix for this scenario.
We haven't assigned a CAR (corrective action release) ID for this yet but will be soon.
Not too sure if a CAR has since been issued but Support will be able to advise you. Hope this helps.
Just to add to MK's reply, from your post I wasn't aware of your version for Web Services but ours is V9.0.00.000 and therefore the patch provided to us by Support is PACOB.9.0.00.006.