Error Deserializing ViewState

Hi All, I am getting an error "Error Deserializing ViewState". Ichecked other discussions but it does not help. I am using a multiselect web block linked to a list box (combo box). As soon as I try to iterate on the list in the any screen action, I get this error:

Please help

The object reference not set to instance of an object can happen if you're in a list and iterating on that list from within it.

Be sure any operations you perform on the list in question happen on a duplicate, not the one you're currently using.

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