0Fixing wordpress visual editor

I’ve had an issue recently where the word­press visu­al edit­or stopped work­ing. The code edit­or worked fine, but the visu­al but­ton just did­n’t do any­thing at all. I knew it was­n’t a serv­er set­ting because it impacted 2 of 5 word­press sites on the serv­er, but the oth­ers worked fine. I tried delet­ing and copy­ing fresh cop­ies of “wp-includes” and “wp-admin” in case some­thing had got cor­rup­ted but this did­n’t resolve the prob­lem either.

I also tried cre­at­ing a new user, which loaded with the visu­al edit­or activ­ated. How­ever, as soon as I switched to the text edit­or I got stuck and could­n’t switch back.
I tried dis­abling all plu­gins, and chan­ging theme, but neither of these helped, although this might be due to extens­ive use of cach­ing on the live site.
Even­tu­ally I found a sug­ges­tion on WPGuru to add a set­ting to wp-config.php which did the trick.
define('CONCATENATE_SCRIPTS', false);
This sug­gests that one of the scripts on the 2 affected sites isn’t oper­at­ing cor­rectly so I will try to track this down. The strange this is that it can­’t be a built-in script because I re-copied the wp-admin and wp-includes folders, but it also should­n’t be a theme or plu­gin as I switched to a default theme and dis­abled all plu­gins! Very strange!
When I have time I will invest­ig­ate fur­ther to pin­point the prob­lem script and will update this post accord­ingly. My best guess at the moment is that it is hid­ing in the wp-con­tent dir­ect­ory struc­ture somewhere