admin account locked out of web interface when changing "Admin Settings"

[grigutis]

To reproduce:

1. Login as the admin user.
2. Navigate to the Admin Area > Utilities > Admin Settings
3. Change the sender email.
4. Logout and try logging in again.

This is what appears in the apache error.log:

PHP Fatal error: Uncaught Error: Call to undefined function simplexml_load_string() in /usr/local/bin/BlueSky/Server/html/admin/pageHome.php:116\nStack trace:\n#0 {main}\n thrown in /usr/local/bin/BlueSky/Server/html/admin/pageHome.php on line 116

https://github.com/logicnow/BlueSky/blob/d435f662794f8b6dbbe54f5bb4883a64ac03fb97/Server/html/admin/pageHome.php#L116

[grigutis]

After looking at the code, I'm guessing that uncaught error isn't related, but I could be wrong.