Sourcing Cockpit: Document still being processed in background

Dear experts,

we are facing the following problem in SRM 5.0 extended classical szenario.

In transaction Sourcing Cockpit when I click on create PO I reach the "Sourcing - Document Preview" screen, where I can click on button "Process PO".

Everytime I click on button "Process PO" I get the error message "Document is still being processed in the background; try again later".

PO is created but in status HELD. I have to go to transaction "Process Purchase Order (BBP_POC)" in order to create PO so that status changes to ORDERED.

There are no entries in table BBP_WFLOCK.

Thanks in advance and best regards

AD