Hello Desti,
For solving this issue, it is important to note that the Codepage of Front End should be the same as the one configured in the SPRO settings for the Webservice of the Catalog.
You have tried to set UTF-8 to resolve the issue, however still the error occurs.
Could you check with Txn SNLS, which is the "Codepage of Front End" then set the same in SPRO settings.
Also, please note regarding the issue you face when you don't set BYPASS_INB_HANDLER parameter in call structure:
Could you try to add the following parameter in the SPRO settings:
returntarget = _parent.
If issue still persists, share a screenshot of your SPRO settings and kindly confirm your external catalog supports OCI 4.0
Regards,
Bhanupriya