no entry is usually not an issue, this is creating itself with a first successful execution.
I see here much to much discussion where people want the settings equal for everybody and then write programs to do so, which could have been then a source for the issue.
Take Williams advice and debug to see which values are being processed when it comes to the problem, an exit or BADI should actually bring the same results in QA and Prod except it is already worked on and not yet transported, but I think there is still a wrong value that causes this issue.
Have you already tried with a different PO, just to make sure it is not PO related?
Btw, there was a similar discussion where the user posted his solution last week Not allowing to enter Service Entry Sheet (ML81N) and showing the error no SE559?