I’m having a strange issue. I’m running SS 5.8 on IIS 10, .NET 4.7.2 (I’ve also tested on 4.8, issue happens there as well). On rare occasions a response from Service Stack is mangled, essentially it looks like the contents of a previous request. I initially suspected a caching issue or some sort of thread-unsafe thing I did, but I was able to reproduce with a python client and a fairly minimal solution running on IIS Express.
I’ve published the solution at https://github.com/almostjulian/ConcurrencyRepro, including a small python client and some batch scripts to generate load continuously. On my machine, the python client will error out on some of the requests (which i’ve color coded in red so you can see) with the some of the contents of a previous request. On rare occasions the response will be so mangled that the python client can’t even deflate the response.