On mine, PopupDomination was causing the error. If you have that maybe disable it and see if that helps.
Unable to create directory //wp-content/uploads. Is its parent directory writable by the server?
Look at the leading slashes. Delete them and try again. (Settings-Miscelleaneous)
I had my host reconfigure my site and it still didn’t work, then saw this post and looked back at mine. I had a leading slash that I deleted and it worked.
