I have seen several posts that talk of screen flashing but my problem in addition to the flashing the entire Roku Ultra freezes in the flashing mode and I cant reset it or go to another page. I finally did a factory reset (hold the reset for 30 seconds) and it stopped and I got the ROKU startup app but I also lost all my programming and PWs etc. Is there a way to get past the flashing w/o doing a factory reset?? I do have the newest LG tv (75NANO90UNA) that can handle 120Hz refresh so that is not the problem ??
A factory reset puts the Roku into a new-out-of-the box status requiring a new device setup, thus removing any info you've entered to this point.
Next time you could just try a system restart via Settings .. System .. System reset.