SAP Note 18625 - ME91 Purchase order only urged three times

Symptom:

When an urging letter has been sent three times for a purchasing document, no further urging is possible.

Cause and prerequisites

This was not supported.

Solution
A modification is available. An enhancement is contained in the standard system as of Release 4.5A.
If you implemented Note 81176, implement the modification for Release 3.1I.