Ok, it took me pulling from some IT knowledge as I had this issue at work after the 1803 update. After the update some of our in-house created software for the place I work stopped working as it was labeled a virus and I started getting the same errors I got when I opened a trainer from MAF this morning.
The issue is related to the way Microsoft widened the scope of what they consider to be a virus or harmful file. This is nothing more than a false positive created by Windows Defender. To get around the problem for now, you'll have to turn off Windows Defender. Unfortunately, they don't give you an easy way to do this, so you'll have to go into the Group Policy Editor in order to do it. If you're not comfortable with editing this kind of stuff, don't do it, I also wouldn't recommend it if you don't run any other kind of AV program.
Hit "Start" and type "gpo", you'll see "Edit Group Policy" pop up. Select that and navigate to the following:
Computer Configuration > Administrative Templates > Windows Components > Windows Defender Antivirus > Real-time Protection
Once there, the double click "Turn off real-time protection" and choose "Enabled".
Also, make sure "Scan all downloaded files and attachments" is disabled.
This will allow you to download and use MAF's trainers once again.
DISCLAIMER: This will also completely disable any protection from Windows Defender Antivirus. Not recommended if you have no alternate AV protection. This is also only intended as a temporary fix until a better solution is found, I just wanted to help everyone get back to using MAF's trainers. Good luck!