We performed an additional investigation with client`s project and we found that the for each statement in the client project does not dispose all controls. It should be used the following code snippet: 
while(panel1.Controls.Count > 0)
{
    panel1.Controls[0].Dispose();
}

More information how to dispose controls can be found here: http://stackoverflow.com/questions/17627038/foreach-control-c-sharp-skipping-controls and http://stackoverflow.com/questions/1511047/how-do-i-dispose-all-of-the-controls-in-a-panel-or-form-at-once-c-sharp