local device name already in use

I have a portable on the network with 2 mapped drives. They work fine in the office. I take the portable home and logon to my user profile. I then start the VPN software and am able to ping all devices on the network at the main office. However when trying to access one of the mapped drives, when clicking on them I get “Local Device Name Is Already in Use”.

If I disconnect the mapped drive and try to add it again from home, it prompts me for my network username and password. When I try to put them in it tells me I can’t use this login because its already been used to logon to the portable. I have to remap using someone elses username and password. Its very frustrating. Any ideas?