Error ME006 when releasing multiple POs in ME28

Hi guys

Hoping someone can assist with this query.

When users attempt to release multiple POs at once via ME28, they get error message ME006 - 'User *** already processing purchase order ***'. The first PO is released but no others.

I've checked SM12 for lock entries but there is nothing there. The PO on the error message is definitely not open in another session. Has anyone ever come across this before, and if so, what's the solution?

We are using ECC6.0, but this issue first arose in 4.6C.

Many thanks

James