Hi,

Thx for the answer.

I found a solution to this problem at: http://www.sharepointblogs.com/ssa/archive/2007/10/16/null-is-null-or-not-an-object.aspx

The reason for this problem was that the default postback option of the control changing the hiding condition ("Only when necessary for correct rendering of the form") didn't postback correctly. After changing the postback to "Always" the error vanished. Actually I still don't know why this happens with grouped controls only, but at least this fixes the error.

Regards,

Bernhard