I have a user group called Content Loaders with limited permissions. After creating a test user in this group I got an "An unknown error occurred" message every time I navigated anywhere in the CP or saved a change (though both were successful). I started adding back permissions and when I re-enabled General > Perform Craft CMS and plugin updates, the errors went away. Not sure if this is a configuration issue on this new staging server, but everything else seems to be working. I can't find any posts about similar issues. Obviously I don't want people doing data entry to be able to perform Craft updates! ;) Anyone have a clue what might be causing this?
Possibly related: even with an admin account checking for updates just shows the spinner image endlessly. I have no problem checking for updates on my local install. This is running on DigitalOcean (with ServerPilot), Ubuntu 14.04.4 x64 with PHP 7.
The only errors I saw in craft.log (which I cleared before doing something that caused the cpanel error to appear) looked like this:
2016/07/31 03:45:43 [error] [php] include([snipped path]craft/plugins/commerce/vendor/composer/../guzzle/guzzle/src/Guzzle/Http/Client.php): failed to open stream: No such file or directory ([snipped path]craft/app/vendor/composer/ClassLoader.php:412)