Themes will not update in some domains

OK, bare with me…this is really weird!

I and my server engineers and the theme developers have been fighting a really weird issue for the last 11 days and all of us have no clue where to look now and we need some help….so the WP community gets this one!

I maintain over 50 client websites; some are on my own servers, some are on client servers. I use a variety of themes from commercial/premium themes (mostly) to a few free themes and a couple of self-built themes. This particular problem concerns a premium and well respected theme called U-Design.

My baseline here is that I have 6 licenses for U-Design. Five of these websites are hosted on my servers and the other is hosted on a client server. Now here’s the weird part… I can update plugins on all these sites. I can also update 3 of these websites’ themes without issue but the other 3 I cannot.

The websites I cannot update are all hosted on my servers.

The REALLY weird part is that when my server engineers OR Andon at U-Design logs in to these sites they CAN update the theme!

I have deactivated ALL plugins and attempted a theme update (fail), I have switched theme and attempted the update (fail). I have tried the update from all possible locations with the Admin dashboard (all fail). I have duplicated all settings (php.ini, wp-config.php, .htaccess etc) taken from the 2 sites on my server I can update and attempted the update (fail).

So I have covered all the basics (much of this was done before the server engineers were involved or U-Design were asked for help). So in my mind I am 100% convinced there are no conflicts of any sort – partly because I have deactivated all plugins and partly because I can update 2 almost identical sites on the same server. Because the engineers can update then that either points to my browser or ISP.

So the next steps were taken. I installed Open DNS (no change), I used a VPN connection using CyberGhost (no change), I logged in and used my iPad (WORKED!), I installed Maxthon and a couple of other less well know browsers and tried (all failed), I used a different PC but on the same internet connection (fail), I have installed a new WP install on a different domain and cloned the site and then tried an update (fail), I’ve created a new Admin account and tried updates from that (fail) and before each attempt I have been clearing cache and history…..

N.B. By (fail) I mean the update appears to start but the page doesn’t refresh with the usual message – even after a decent delay (2-3 minutes) so I am forced to FTP and delete the .maintenance file.

So I think you’ll all agree I have tried everything but there is too much weirdness going on here…I can update manually via FTP of course or take the easy option and use my iPad to log in and update but I’d rather find out what on Earth is stopping me update these 3 sites.

Anyone got any idea where to look now?