When ads are due to start the screen goes black for about 20 seconds? It never used to?

Under "Settings > System > Advanced system Settings > Advanced display settings" does your Roku have an "Auto-adjust display refresh rate" item? If you don't have this setting, the rest of this post doesn't apply to you.

If this is enabled, the Roku circuitry sends the signal output at the refresh rate (frame rate) used by the original source material, commonly 24 fps (frames per second), 25 fps, 30 fps, 50 fps, or 60 fps. Every time the source frame rate changes, the TV must adjust its display to accommodate the new frame rate, which on many sets results in a short blackout or other display disruption that some find objectionable.

This can happen when starting/stopping new programs, and when going in/out of commercial breaks that have a different native frame rate than the programs they are inserted into.

If "Auto-adjust display refresh rate" is set OFF, the Roku converts and sends everything at 60 fps so the TV doesn't have to adjust on the fly and you don't get those frame rate transition disruptions.

The tradeoff here is that movement of things on the screen for non-60 fps sources may not be as smooth with no frame rate adjustment, depending on whether your Roku or your TV does a better job of refresh rate conversion. You have to decide whether this change is worth it to you.