After spending some time looking into this problem, I think I've found a work around when MEGA freezes during the download process.
I hope the following instructions work for you. These instructions assume that you already have your own MEGA account.
Step 1. When your MEGA download freezes (even when Decrypting), and you've given up on it resuming, select the option
Save to MEGA
Step 2. When your MEGA account window opens, select the CANCEL button next to Save to MEGA (bottom right)

Step 3. You should then see a small Download progress window pane appear, bottom left of your MEGA account window. If it doesn't display COMPLETED, select the
PAUSE all Transfers option and then
RESUME Transfer. Give it a little time to sort itself out.
Step 4. This step should force the continuation of the download / decrypting process, and eventually result in a green Completed (tick).

Note: I am unsure what will happen if you don't have a MEGA Account, but it's worth a try.
Good luck