Have reviewed your install and it's because your host does not have PHP encoded with iconv. This is used for UTF8 conversion during History Logging. The good news is even though you get a blank page your changes are still saved.
There are two ways to fix this. First way is contact your host and have them enable iconv (best approach). Alternative is to perform the below edit and remove the iconv usage (I don't know the dangers of doing this).
IN: /components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/models/table/cbpaidTable.php
ON: Line 426
FROM:
Code:
$v = @iconv( 'UTF-8', 'UTF-8//IGNORE', $v );
TO:
Code:
//$v = @iconv( 'UTF-8', 'UTF-8//IGNORE', $v );