As far as I know, the reason may be that there is an inconsistency within
the customizing data. The table which holds the information about the
action definition (PPFTTTCU) holds also the information about the action
merging. There should be one GUID included with which the merging details can
be found within table PPFTMERGE.
Please check if there is corresponding entry available within PPFTMERGE.
Please make sure there is no inconsistency between tables PPFTTTCU and
PPFTMERGE. The entry within PPFTMERGE which corresponds to the PPFTTTCU entry
should not be missed.