I'm seeing a sporadic "Collection was modified" exception when enumerating the eventKeys enumerable in InProcessMessageBus.WaitForMessages.
The simple fix is to do
var eventKeysList = eventKeys.ToList();
at the top of the method, then use eventKeysList in place of eventKeys.
I also get this error a lot, and the fix mentioned above works.
This is also related: #388
Note that I was also having other issues, with my clients not receiving some messages issued by the server (it was very random).
After applying this fix, all messages now arrive properly at clients.
Fixed the thread safety problem with groups and the Connection.cs
Fixes #388, #537.