App from Hockeyapp won't install even though Unknown sources is allowed

I am getting a weird error, only in 1 of my phones. I am using Hockeyapp to distribute and test apps and it works fine in every other mobile device. In Oneplus 3 (A3003; Android 8.0.0; OxygenOS version 5.0.6) though, when I try to install an app from Hockeyapp I get the message that I need to enable 'Unknown sources'. After I press APP SETTINGS I am taken to the unkown sources screen where Hockeyapp is selected, just like the image below. " class="img-fluid rounded" style="max-width: 100%; height: auto; margin: 10px 0;" loading="lazy"> If I go back in Hockeyapp and try to install the app again, I still get the same error message. I tried to clean install Hockeyapp, change manually the unknown sources but nothing changes. I repeat that this only happens in this Oneplus device. How can I solve this and install the app that I want from Hockeyapp? **Edit:** I uninstalled Hockeyapp, installed it again and explicitly granted that permission. I still get the same error!