Just had this start to happen to me as well. Funny thing was it was working fine for weeks, then suddenly all my thumbnails display as grey - you can sort of see the game behind the grey sometimes.
I have been doing different things with my channel, such as adding a text overlay, and upping to 60fps. I've also enabled CBR as per the Twitch FAQ which recommends it (and my connection can handle it).
I've tried dropping the text overlay, that had no effect. I haven't tried dropping the 60fps yet, so I'll try that next. If that still doesn't work, will drop CBR as well as all the others. Then my stream will be back to the default settings practically and I'll see if the thumbnail comes back.
I've noticed a few other grey thumbnails in the list of streamers, asked one and he was using OBS. So it is still happening.
I'll send in a ticket to Twitch to just add my voice to those asking them for a resolution - they really should be able to take a look at one of these streams at the time and just see on their end what's going on so we can fix it.