It might be an issue similar to what I have experienced with the same version of Dropbox on a Windows PC, after updating the OS. Trying opening dropbox.com in your browser to visit your Dropbox account; then click on the top right icon of your account personal icon, and click on Settings. Then open the Security tab and scroll down to see which Devices are linked to your Dropbox. In my case, I found that my Windows PC was listed several times, perhaps because upon each update it had been registered as a new one. Disconnect all the duplicates (do not tick the mark suggesting to delete the content!); if in doubt about which is the most current one, just disconnect all Macs registered to your account. The dropbox will disconnect on your Mac, you will have to reconnect and hopefully the syncing should should start working again. Anyway, if you have lots of files, it may take a long time anyway, so I recommend being patient, the first syncing may really take several hours. Hope it helps!
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