Silly question but did you install Brave with full Windows admin rights (not just as your local user)? Just a random thought.
Also are you trying to sync with an iOS device? If so see this thread (it may be a contributing issue): For well over a month now sync is completely broken iOS devices - plus other bugs with sync on iOS too. (iOS) If you have iOS please can you read
Otherwise what devices are you trying to sync with? It might help someone else to comment on possible cause or fix?