The page is loaded correctly at first but during postbacks in many cases it ends up with error saying that some of DropDowns has a SelectedValue which is invalid because it does not exist in the list of items. In this post I have described the same problem and the solution was to specify the TopMostItem. However - all of dropdowns has the TopMostItem set up!

The page works correctly when I remove these 3 dropdowns. I'm really confused by this behavior and cannot find the cause of this error.

I can email You my project (I cannot post it to whole forum) if necessary.