Symantec antivirus is treating the installation of your product as a virus (the last one - I just tried to download it and run, and faced with what you can see on the attached screenshot).
The version which I have now (18.104.22.168 64bit) still works, but I think it is worth to investigate from your side, otherwise you will loose users...
I don't see anything Greenshot related in the screenshots, it says something about firefox?
And maybe you didn't download Greenshot from our website?
Maybe you installed Greenshot directly from the download directory, this can also be a risk. Although we use an installer (Inno-Setup) which should prevent side-loading (Dll-Hijacking) we cannot guarantee anything. In general, not just for Greenshot, you should always download your applications and than copy them into a clean directory before starting the installer.