I had exactly the same problem when updating to Version 10. I didn’t know, why it happened, but I knew how to solve it. Probably not the best solution, but it’s working.
I changed the Host in the mysql user table from localhost to % and the Host in the mysql db table to %. Please note, if you don’t know exactly what that means, I can not suggest this as a solution.
After that changes, flushing the privileges via SQL query and restarting mysql, it worked.