If you are using a static variable in a web application, you have to synchronise all access to it. The reason for the error is that two users are adding items at the same time. You are lucky that you got such a descriptive error message, you could just as well have corrupted the HashTable and got some internal error in the HashTable code... Use a List instead of a HashTable. You are only using the key of the HashTable as an index anyway. Instead of copying the items one by one into a new collection, just keep them in the collection where they are.
Despite everything, the person most likely to be fooling you next is yourself.