The same issue is back. This recurring problem seems to be on Roku's end. It self-corrected after days last time (explained above).
I added a new video to the channel last night...exact same type of thing I've been doing for years (just like when this problem came up a few months ago) and the channel now shows errors again!
1 item has an error: No valid items were found in the feed: validate09/18/2019 10:33AM
-->The error is 137060.pending.en99 items have error:
Thumbnail must be 16x9 aspect ratio, at least 800w x 450h, and either in JPG or PNG format.
-->ALL images are 16x9 and over 800w x 450h jpgs (and this error didn't show until the latest upload)