In ECP go to tx st05, you should only need RFC Trace but the others can't hurt just provide more logs.
Now click on "Activate Trace with Filter" and it will allow you to select a user ID, I would start with the account that is being used in the web UI of esh_cockpit along with the ID used for Trex to talk to ECP. You may need to run 2 traces when trying to prepare the index to catch what ECP might have an issue with.