current logical threads increasing / thread stack is leaking

Monitoring my .NET app in Performance Monitor I can see the .NET CLR LocksAndThreads / # of current logical Threads is increasing steadily (currently 293) over time which indicates the thread stack is leaking.

I can find many articles which tell me this is the problem but nothing that tells me how to find the cause - so where do I start? Can Windbg tell me where the problem lies?

This is my performance monitor over 3hrs telling my current logical threads is 150:

And this is the output of the threads window, which doesn't tell me much because I can't access their call stacks - they are mostly marked as [unavailable] or [In a sleep, wait or join] | [External Code]:

Unflagged       141024  124 Worker Thread   <No Name>       Normal
Unflagged   >   0   0   Unknown Thread  [Thread Destroyed]      
Unflagged       136272  2   Worker Thread   <No Name>       Highest
Unflagged       133060  7   Worker Thread   vshost.RunParkingWindow [Managed to Native Transition]  Normal
Unflagged       136952  10  Main Thread Main Thread [edited].Program.Main   Normal
Unflagged       134544  9   Worker Thread   .NET SystemEvents   [Managed to Native Transition]  Normal
Unflagged       136556  11  Worker Thread   Worker Thread   [edited].MessageService.ProcessJobs.AnonymousMethod__0  Normal
Unflagged       141364  113 Worker Thread   <No Name>   [In a sleep, wait, or join] Normal
Unflagged       140896  0   Worker Thread   [Thread Destroyed]      Normal
Unflagged       136776  19  Worker Thread   <No Name>   [In a sleep, wait, or join] Normal
Unflagged       135704  20  Worker Thread   <No Name>   [In a sleep, wait, or join] Normal
Unflagged       136712  21  Worker Thread   <No Name>   [In a sleep, wait, or join] Normal
Unflagged       134984  22  Worker Thread   <No Name>   [In a sleep, wait, or join] Normal
Unflagg