I've set the 'data' and 'internal_data' folders (and subfolders) to '777'.
Sometimes I get the error the second I click 'Install addon', other times, it'll happen at any point during the run-deferred process. Usually I have to uninstall the addon and re-install it, often two or more times, before it will successfully complete installing an addon.
Often, when it fails (and I get the 500 error), depending on where it failed, portions of the addon (say, the admin options for that particular addon) will be completely borked.
This is occurring from a completely clean install of XenForo, on a brand new cPanel account, with no other installed software. This is shared hosting, but I've ran XenForo previously on the same server without this issue.
This occurs with or without full friendly URLs enabled/disabled, and with/without the appropriate .htaccess in place.
Any suggestions/tips appreciated, this is driving me bananas.