I'm having this same issue of thumbnails not showing up. I recently installed Dropbox on a new computer and am having this issue. Please message me directly if you can help as you did others, moderators!
What would be helpful is for them to finally resolve it. This has been going on for a while now and I am not sure if I will hold off much longer before choosing an alternative.
Currently the only offering has been to suggest switching the folder from local to online then back again. Which in may case can mean doing this several times a week, and that is before I fire back up after lockdown and start shooting several times a day.
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