'Add Assets' button still causing problems

I'm still having an issue with this (https://forum.grabaperch.com/forum/07-15-2014-add-asset-button-missing?page=1#reply-2031). Well, not me: my clients are. The fix in 2.5.4 worked for me but it's still breaking on the client's end. I got them to clear cookies in case it was something to do with their cache, but no luck. I then asked them to log in outside of the network they were on with a different machine but still no button.

It's working fine for me. I've even tried it on 3G on my phone and (although the interface is a bit broken at mobile) the '+ Add Asset' button appears ok. Bit of a weird one…

Checked there are no permissions I need to enable for them to see the button, but can't find anything obvious.

They're getting really frustrated and I'm getting a bit embarrassed. Am I able to downgrade to 2.4.9 without any consequences? If so, where will I find a copy of 2.4.9? I guess I could just copy the core folder from the upgrade to 2.4.9 commit from my own site's git repo.

Also, I hope this is the right place to post this– I've been trying to get in touch since Friday lunchtime :( (admittedly, my second attempt on Monday was on an incorrect forum thread, as Rachel pointed out)

Perch: 2.5.4, PHP: 5.4.29, MySQL: 5.5.38, with PDO Server OS: Linux, cgi-fcgi Installed apps: content (2.5.4), assets (2.5.4), perch_blog (4.1), perch_events (1.9), perch_gallery (2.8.5), perch_backup (1.2) App runtimes: <?php include(PERCH_PATH.'/core/apps/content/runtime.php'); include(PERCH_PATH.'/addons/apps/perch_blog/runtime.php'); include(PERCH_PATH.'/addons/apps/perch_gallery/runtime.php'); include(PERCH_PATH.'/addons/apps/perch_events/runtime.php'); ?> PERCH_LOGINPATH: /cms PERCH_PATH: /var/www/vhosts/skeltonnewbyhall.n-yorks.sch.uk/httpdocs/cms PERCH_CORE: /var/www/vhosts/skeltonnewbyhall.n-yorks.sch.uk/httpdocs/cms/core PERCH_RESFILEPATH: /var/www/vhosts/skeltonnewbyhall.n-yorks.sch.uk/httpdocs/cms/resources Image manipulation: GD PHP limits: Max upload 128M, Max POST 8M, Memory: 128M, Total max file upload: 8M Resource folder writeable: Yes SCRIPT_NAME: /cms/core/settings/diagnostics/index.php REQUEST_URI: /cms/core/settings/diagnostics/ DOCUMENT_ROOT: /var/www/vhosts/skeltonnewbyhall.n-yorks.sch.uk/httpdocs HTTP_HOST: www.skeltonnewbyhall.n-yorks.sch.uk