I did open a ticket about undefined thumbnails after the last big update.
I had to explain the issue several times to different members of support staff, almost 2 weeks of messages going back and forward, just to state that thumbnails aren't working.
They made me feel like never ever in evernote history someone had reported a similar issue. I had to send several screenshots + screen recordings + .enex files + logs + remote session (it started 35 minutes after the scheduled time), and in the end, their official answer about the issue was:
"We are now aware of the issue, for the moment is not possible to create thumbnails from high res images (seems like out of millions of evernote users i was the only one using high res images ).
The problem has been reported to engineering team, however we can not estimate when it will be addressed, or assure you that we will actually take care of your request (seems to me like it was requesting a new colorful theme or a high demanding feature, and not a very basic fix )."
So, if you are thinking about going directly to evernote support for help about this same issue, save your time.
