A few suggestions...

First place to look is the clear events somewhere on the modal form as Dragan mentioned.

If the modal form is small, try creating a new one, with not much on it and see if callinf the nwe modal form behaves the same way.

Another gotcha is the possibility of defining an object in the modal form that is the same name as an object needed for the main form.. Releasing the modal form removes the object and the main form doesn't like that.

Hope this helps.