I had a personal Dropbox account and was invited to join a shared business workspace. I chose the option to combine accounts (because the installer assumes that your existing account must become the business one and you should create a new personal account. There is no offer to do it the other way around!)
But the installation process stops with a message box titled "Can't sync Dropbox account" and content like "Your account can't be updated to the new *** shared workspace. To continue syncing your Dropbox account, Close the following files or folders;
C\Users\[username]\Dropbox\". The details box just says "Couldn't rename C:\Users\[username]\Dropbox\".
I had nothing open from my Dropbox folder, and I was not in the mood for unlinking accounts and re-syncing, etc. It sounded like a horror story for any that went down that road.
The solution: Close all programs, exit your anti-virus software, then click the re-try button and voila it works! (My anti-virus is Trend Micro Maximum Security)