Hello Golo
Similar, to the issue you had, I have the identical issue.
I call the Start-Screen directly through PFCG role. No portal is envolved at this time.
I am using a standard lauchpad (not customised). I do have a url
/sap/bc/ui5_ui5/sap/HRESS_EMP_PRO_G/index.html in the role ESS MYINFO. I still get a blank screen.
Can you please share how your issue got resolved?
The difference I see between ESS MENU role and ESS MYINFO is of the system alias they use.
ESS MENU - SAP_ECC_HumanResources
ESS MYINFO - SAP_HCM_PROXY
The RFC connection as suggested by SAP has been created as SAP_ECC_HumanResources.
How about SAP_HCM_PROXY? Could you share some information with regards to the issue.