Permanent license - Ticket not found

I've set up a license server on the office LAN and installed ReSharper on my laptop. Then, I requested a permanent license because I sometimes work offline or in the field and would like to use ReSharper also then. This worked fine for a while, but now I get an error message when starting MS Visual Studio 2010: "License server response: Ticket not found". All controls in the ReSharper License Information dialog are disabled so there's nothing I can do to reset this. Do you have any tips for me on this issue? I'm running ReSharper 6.1.37.86. All other computers using floating licenses work fine.