Hello Hilary,
thank you for the links. These links indeed describe both the problem I have encountered. The problem comes with the installation of the "June Cumulative Update package" (NOT with Service Pack 1 only !!!). In short words the problem appears with a postback in the form. In my form it is the postback from the attachment control. I don´t know the exact problem but Microsoft has already confirmed this to be a bug in the "June CU" with the "Form Server".
I have fixed the problem now in the following way --- though this is not "best practice" but there´s no other solution until MS publish a hotfix...
To fix this issue you have to change the "FormServer.aspx".
This file is located in:
C:\Program Files\Common Files\Microsoft Shared\Web
Server Extensions\14\TEMPLATE\LAYOUTS
In this file you will find the line:
<body
runat="server" id="PageBody">
After this line you have to insert the following line:
<SharePoint:FormDigest
runat="server"/>
Now everthing works fine and the attachment control works as expected...
Regards,