"Open in server" is like using an AccessPoint client inside your desktop. You'll need everything that a regular browser user needs when visiting the AccessPoint, like a CAL from the server, permission to open the document, listing in section access and the likes.
Check these things and try again.
CALs for accesspoint clients can be used on two platforms max. If a Named CAL has been leased already, you only have one platform left for visiting the AccessPoint. Every subsequent visitor with the same CAL will kick out the previous visitor ("Session lost")
Note that on the same platform, you can start as many sessions without exhausting the number of platforms/MAC addresses.
Can you find a system in this behavior? Try with one CAL for which you can control the use and the location where the license will be activated.