If you can't login and start getting an error similar to...
PHP Warning: base64_decode() has been disabled for security reasons in .... /store/classes/Rijndael.php
It is caused because your IP Provider has disabled the base64_decode() function in your server.
They must remove the base64_decode string from the disable_functions in the php.ini* config file.
I created this short PHP test program and put it in my store directory while they tried to fix the problem.
<h2>Test Code For PHP base64_decode()</h2>
If the base 64 function works properly, you should<br>
see <i>"This is an encoded string"</i> printed out.<br>
<br>
You are attempting to run this code...
<div style="background-color:#def; width: 450px; margin-left:25px;">
<xmp>
error_reporting(E_ALL ^ E_NOTICE);
ini_set('display_errors', '1');
$str = 'VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw==';
echo base64_decode($str);
</xmp>
</div>
<?php
error_reporting(E_ALL ^ E_NOTICE);
ini_set('display_errors', '1');
$str = 'VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw==';
echo base64_decode($str);
?>