... if think, this is be the first half of the solution. :-)
The error is a combination of request_buffer_size and max_concurrent_long_request.
So the most policies containing file types with a request length of 10000, but there are some file types with a length of i.e. 100000 or "any".
It would be nice to know, how many requests are bigger than the request_buffer_size. Perhaps in a scale like 10000-20000, 20000-40000 or something like that.
This would be a good way to setup a value befor you get an error message.