I have a user that cannot drag & drop an email into a specific card

[Athena Kennedy]

Hello, I currently can drag and drop an email straight into a Trello card that I have created. I recently added a user onto one of my boards that I created. He used to be able to do that and not it is not allowing him to. 

When he tries, it just gives him an X when he hovers into the card when he tries to drop the Outlook email inside a card. 

How to I fix that to allow him to drop an email inside the card?

All the other users are able to do it, just not this one.

Any help would be appreciated. 
Thanks!

[Gaurav Kataria]

Athena, what browser is this user using? Could you ask them to try it again on the latest Chrome browser.