We were experiencing a similar issue. The note from Michael Volmer was also checked and we did not encounter any issues with regards to that in our SolMan.
The solution, which may not be applicable to almost anybody out there, is that we had inactivated some of the SAP transaction types, like SMMJ, SMHF, SMAD, etc. to make sure that:
1. nobody was going to use the SAP standard ones, or that
2. our Z transaction types were not going to divert to the S* ones internal/externally for missing copy configuration on our side.
This is a kind of silly approach as things can be controlled through SAP security, but that was a safe lock we used at the beginning of our upgrade project, to avoid having to deal with SAP Security at an early stage of it.
SPRO > CRM > Transactions > Basic Settings > Define Transaction Types.
Regards,
Juan Carlos Garcia