I'm using NH 4.1.0. I'm unable to reproduce the error: it seems to happen randomly, having show_sql set to true I get an exception that deadlock the application:
Windows Server 2012 R2
For this issue to happen, and as the error message implies itself, it seems likely to me the session has ended-up shared among many threads, which is not supported. The session is not thread safe and must be used by a single thread. This includes implicit usages due to proxy initialization. (For example, when accessing an association, by default it is loaded on demand.)
Please add on the above linked GitHub issue any additional elements which would help reproducing the trouble if it can be done without a multi-threaded usage of a session.