Total white screen / blank page (after opening "Plugins"-menu)

I’ve copied one page to another path (staging to productive) as I have made it already several times to have testing-versions of the live-version. Usually this is going fine. But this time I encounter major troubles.

What I’ve done:
– copied all files to the new path
– imported the previously exported database, urls changed before importing

After I’ve made the final modifications to go live, I had a blank page.
To cut the long story short after 2 days of troubleshooting: the moment I klick on the word “Plugins” in the admin-panel the site dies. I can only bring it back by dropping all tables and re-import the db (probably it’s a specific table causing the problem but I did not figure out which one yet).

I’ve activated debuging, but nothing is shown (page remains blank, no file is created).
error_reporting(E_ALL); ini_set('display_errors', 1);
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);

I’ve re-copied all files, no difference.
If I remove all files from the plugin folder I get the main-menu but nothing more.
I’ve removed all plugins not needed by any manner of means (eg. I can’t skip WPML) before klicking “Plugins” the first time, I see the message “..was deactivated”, again next klick gives blank (and these plugins where used in its combination for several months, I don’t think a particular plugin/combination is the issue)

I’ve increased memory
define('WP_MEMORY_LIMIT', '200M');
ini_set('memory_limit', '200M');
memory_limit = 200M in php.ini

The page is at http://ubiquarian.net, but currently it’s pointing to the staging-db so the error cannot be seen.
The source-code while blank is

<script type="text/javascript">
<!--
jQuery(function($){
	jQuery("#ssh").click(function () {
		jQuery("#ssh_keys").show();
	});
	jQuery("#ftp, #ftps").click(function () {
		jQuery("#ssh_keys").hide();
	});
	jQuery('form input[value=""]:first').focus();
});
-->
</script>
<form action="" method="post">
<div class="wrap">

Anyone a clue what is going on?
Any idea what I could do or try else?

Many thanks in advance!