I have this same issue however deactivating all plugins, changing permissions etc will not fix it. My problem is the blue set image button will not activate once the blue button has been clicked. Reverting back from 4.8 to 4.7 wont help. Tried other themes no help. Any suggestion would be appreciated. Everything works perfect right up until you click the blue button. This worked fine for months only today stopped working. Have run out of ideas.
