False positive AV in URL/Website

When you access "https://shop.meyco.eu/main/" there is no download. So the site will show clean in all the virus testers. The AV check is done only when there is a file being downloaded - and I don't know what file you are trying to download from that website. 

It seems that the FortiGate is doing its job, but most importantly is to have the most recent FortiOS and AV signatures up to date - these are periodically changed and must be updated.

We only open the Website. But the error message appears directly. We test it with 5 different Clients. But the behavior is identical
FortiOs is up to date (v7.4.4 build2662)
AV Definitions are Version 92.04824

Certainly something is wrong in your policy setup/settings. I can access this site without such log/warnings. The only warning I see is that of the certificate. Quick lab test with both versions (7.2.8/7.4.4):

flow-based mode -> check details about this mode. It only checks the packets as they pass, no reassembly. False detections, misidentifications are normal in this mode.

or something else entirely. Start with a new policy and remove all your customizations - add profiles one at a time (default profiles). Not lastly, try Firefox - Chrome may cache some certificates/sites,etc

In this case it’s not complaining about the mining software itself. Read carefully where it says “safely aborted connection”. It’s blocking the connection to that url. If you are certain that the url is correct for your mining pool then go ahead and add an exception.