It looks like an error from the SAP server. Also, did it work before with MxAdmin? May be you can try with an actual admin account instead of MxAdmin.
Response code 500 generally means “internal server error”. Most likely something is going wrong in your backend service’s logic causing an error response to be returned.
I would suggest checking all your parameters for the service and if all is correct then try debugging the service on the backend to find the issue.
-Hope this helps
Definitely looks like an error in the backend service you are calling. In details it bring us something like error on SAP_Transaction and show us a service.sap.com address that may have some more tips on what's going on.
Try to call that SAP service directly from another software like "Postman" to check if error persists.