I think you'll have to be more specific to get a definitive answer but "cannot amend closed or cancelled activity" is a valid condition that it would seem the web service isn't handling correctly ....
When activities are in a certain state they can no longer be amended (I believe it may be possible to change the activity state, amend them and change it back).
I think you need to look at the code of the web service or discuss it with whoever wrote it...