Same problem here (Dropbox v111 - the lastest stable to the date). On the second computer, no problem with Dropbox v112 (early updates enabled).
It's apparently a problem on Dropbox side since they have release the version 112 fixing this issue: https://www.dropboxforum.com/t5/Dropbox-desktop-client-builds/Beta-Build-112-3-254/td-p/476277
If you cannot update to Dropbox version 112, you can temporarily set Dropbox client to ignore in SSL/TLS filter.
ming gave kudos to RJanata in Dropbox can't establish secure internet connection
