I've looked this error up and tried fixing it for quite a while but am still stuck.
After reading, I learned to try adding the following to my page directive: ValidateRequest="false"
This didn't work, so I tried adding it globally in web.config, like so:
<pages validateRequest="false" />
That didn't work either.
For testing, I was just trying to play the MP3 from my browser by typing in the URL, like so:
This gives me the dangerous request path error.