I have enabled the user exit EXIT_SAPMM06E_012 and when I click on Header Detail of the contract, I got the following dump error: DYNPRO_NOT_FOUND, Screen does not exist.
Program "SAPLXM06" tried to use screen 0201.
The screen does not exist.
It seems that a screen exit was enabled togheter with the user exit. The enhancement is MM06E005.