We have a user on 8.5 Windows client, who has several tasks created each day. Tne user opens each task from the task manager, checks it, marks it as done, closes it, then opens the next one. Quite regularly, doing this causes the client to crash with the following errors popping up:

EndWindow missing: ACTDCLASS

WindowWcdp r1==NIL for window ACTDCLASS

Sometimes it will let her go through several tasks before the crash, other times it will crash after each task. The activity window ActDClass has not been customised. Any ideas?