I started having this problem too, about 2-3 weeks ago.
Using (2) Roku 2’s 2720X version 7.70 build 4135-06
This happens on ALL channels I checked - Netflix, CRTV, Pandora, SiriusXM, Amazon. This seems to point to a software error on a recent release.
When this happens, first the actual thumbnails disappear to be replaced by an icon of 8 circles arranged in a circle, the quickly the are replaced by an icon that suggests a TV set with two “legs” sticking off the bottom of the screen, at an outward angle.
Removing and re-adding a channel does not solve the problem. Performing SYSTEM > SYSTEM > SYSTEM RESTART > RESTART. fixes this.
Reading the thread of this posting seems like there are several others with the same problem. Come on, ROKU. TIME FOR AN OFFICIAL RECOGNITION OF THIS BUG.
C.S.