I'm just returning to the project after a LONG break so forgive me for I'm a little rusty and may not be of much help.
It could be a number of things. Assuming you've downloaded the ClosedXML code and compiled it with the com visible option; then it could be you're loading it from a network shared drive. If that's not the case then I have no clue.
It could be a number of things. Assuming you've downloaded the ClosedXML code and compiled it with the com visible option; then it could be you're loading it from a network shared drive. If that's not the case then I have no clue.