If the user has too many roles assigned, PathTooLongException is thrown when trying to navigate to forum. The source code causing an error is in DNNForum.vb in Menu property:

' Obtain forum menu based on user roles list     
'CP - TEST
If LoggedOnUser Is Nothing Then
    LoggedOnUser.UserID = -1
End If
'END TEST
Dim strKey As String = "ForumMenu." & ModuleID.ToString & "." & LoggedOnUser.PermissionID

If DataCache.GetCache(strKey) Is Nothing Then
    Dim ctlMenu As New DotNetNuke.Modules.Forum.WebControls.ForumMenu(ModuleID)
    DataCache.SetCache(strKey, ctlMenu)
End If

Return CType(DataCache.GetCache(strKey), DotNetNuke.Modules.Forum.WebControls.ForumMenu)

The name of file used for cached data storage includes LoggedOnUser.PermissionID that contains all user role *names*. In my case it is too long since I prefer granular privileges assignment, and DataCache.SetCache was throwing an exception. My hot fix was to disable menu caching. I replaced the entire function code with just

'CP - TEST
If LoggedOnUser Is Nothing Then
    LoggedOnUser.UserID = -1
End If
'END TEST
Return New DotNetNuke.Modules.Forum.WebControls.ForumMenu(ModuleID)

I hope some fix will be applied in further module releases.