Try disabling AMP and see if that resolves the problem.  If it does - you 100% know it is AMP.  If it doesn't, you know you have a different issue.  Then re-enabled AMP.
 
You should check the MX event log to see if it has logged that it is blocking anything.
 
I've add a recent issue with a bug in AMP causing AMP to crash on some downloads.  Because it is crashing rather than blocking, nothing is logged.  I believe it is resolved in 14.53.  If you are running a version below this, try upgrading.