I managed to install 3.1.5 but I had to download the software twice. Each time "Smart" screen detected that it was a threat and I had to say that I wanted to keep the file. The first time I downloaded it Windows Defender then detected a virus, Vigorf.A. I then told Windows defender to allow this so called virus and downloaded the software a second time, again jumping through hoops to keep the file. This time the file downloaded successfully and I proceeded with the install. On starting OpenLP Windows defender again detected a virus - this time Pomal!rfn. Again I went into Windows Security and under Virus & Protection -> Protection History I clicked on the latest threat and told it to allow the so called infected item OpenLP.exe Finally I went in to Allowed threats and removed the allowed threat for Vigorf.A
I have to confess that it felt a bit scary but on the basis that I trust OpenLP not to have a virus I did go ahead!!
Hope this helps.