It appears there's a compatibility issue with version 5.3.1.0 of Blue Iris and Ubiquiti G3 Flex cameras. I have two G3 Flex cameras that both stopped working with Blue Iris early this morning when Blue Iris auto-updated to 5.3.1.0. My other menagerie of 7 cameras (Hikvision, TrendNet, Axis Communications, Amcrest) continued to work.

Directly connecting to the G3 Flex's built-in web server confirms they were running normally and serving video. They are both running in "Standalone" mode to serve video/audio over RTSP. RTSP Authentication is enabled. Firmware version UVC.v4.14.14.67.037e886.190630.1017.

I downgraded Blue iris to version 5.3.0.3 via the "Check for update" dialog (this is a great feature!) and the two G3 Flex cameras started working again. Blue Iris auto-updated to 5.3.1.0 again (oops forgot to turn it off) and the cameras stopped working again. I've since switched Blue Iris to stick with "highly stable" updates.

Just wanted to second this as I had the same issue with my G3s. Hikvision cameras continued to work.