Manual core updates fail (but automatic updates work)

Hi folks,

My blog seems to time out when I tell it to install/reinstall a core update. I guess this worked till about 3.8.0 though it may be related to the hosting, not wordpress so the timing may be coincidental.

I certainly remember that both 3.8.2 and 3.8.3 were installed properly via automatic update, despite me being unable to install them manually.

Possibly related: I have the iThemes Security plugin (aka better-wp-security). I know it can cause problems with some things (and indeed, had to skip some of its recommended settings because they broke stuff). I tried turning the plugin off but that didn’t help things at all.

So… what did I do?

Go to the updates page (wp-admin/update-core.php) and click Install/Reinstall Now.
Wait for about 5 seconds.
Error message: (Not Found. Sorry, but the page you requested could not be found.)

I used the browser’s developer tools to observe the POST to wp-admin/update-core.php?action=do-core-reinstall returns a 404. There’s no useful information there though. I can’t see anything in any error_log files.

I tried setting WP_DEBUG and WP_DEBUG_LOG to true in wp-config.php but that didn’t give me anything useful in the debug.log file.

I’m not sure yet that this is a WP problem but I don’t know how to diagnose this and it’ll become a problem once 3.9 rolls around (since automatic updates only apply to patch releases).

Any ideas?