Now I ran into the same error situation.
...on our test environment Workflow Server (7.5.3001.74). The debugger loads on a web forms project but when i launch the debugger, it goes directly to an error page and log viewer says that "Session not found or timed out". interesting. The url looks valid:
http://localhost:2550/TestProject/StartDialog.aspx?s=pBnT58DuQkSqu9T6l8L3eg
Usually when I have to do something on the server, I log into the Workflow server as an admin and launch the Workflow Manager with "Run as Administrator" option. But this time I logged into the server as an admin and launched Workflow Manager with Run-As -> a non-admin service account credentials.
I did this because the service account has access to a database so when i start the workflow with the service account, the workflow hence and the debugger also have access to that database. But the debugger ran into an exception right away.
I temporarily added the service account to the local administrators group on the server but that did not help. So in my case it has something to do with credentials but right now I don't know what exactly.