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.