Thread:Project:Support desk/web updater gets stuck on environmental checks

I'm trying to upgrade from 1.16 to 1.21.1. I have created a new installation for 1.21.1 and copied my LocalSettings file to it. I'm trying to do the web update (because i don't have command line access). After i type in my UpgradeKey, it goes to a page that shows:

Environmental checks
Basic checks are performed to see if this environment is suitable for MediaWiki installation. You should provide the results of these checks if you need help during installation.
 * PHP 5.3.25 is installed.

...and that's it. There is no next button or anywhere else to go. I'm guessing it ran into some kind of error on whatever other environmental checks it's supposed to be doing, although i turned on error reporting for the mw-config/index.php file and all it shows is "Warning: passthru has been disabled for security reasons in...includes/GlobalFunctions.php on line 2854" which seems unrelated.

Does anyone have any ideas? Does anyone know which files are being used at this step of the upgrade? Maybe i could edit them to bypass whatever it's getting stuck on?