Indeed it works....
This keeps happening to me A LOT (a few times every day). The first time it happened (after I installed Big Sur) I contacted DB help and someone told me to delete the app, re-download and sync again - it worked very well. But soon it happened again... I thought it was a bit 'brutal' to delete and re-install the appevery time (not to mention the first big sync).
It was obvious that the problem was the finder extension - Dropbox worked well apart from that. I found a work-around that works perfectly: everytime the tiny blue/greee/grey icons disappear and no finder extensions, I go to System Preferences > Extensions and there un-click and click again the DropBox extension. Instantly everything starts working again.
It's a bit of a pain to have to do that everytime, but it works.
I don't have many other extensions in my computer, but now I'm trying to identify if the culprit is another extension by having them all unchecked and trying to load them one by one. It's a bit tricky, because the problem appears to be ramdom.
It would be great to have a proper fix, but unchecking and checking the DB extension works!