There is a lot of good information on the Elgg wiki: http://docs.elgg.org/wiki/White_page

If you can repeat the white screen of death (WSOD) and it's not a server configuration issue, it could be a bug and you can report those to Trac: http://trac.elgg.org/

I understand what's written in the wiki, but i don't know how to fix this ... :-( would you help when I give you admin rights?

The wiki says to edit the .htaccess file and change the display_errors value:

php_value display_errors 0

to

php_value display_errors 1

i did this ... but this didn't affect any results ... where shoudl i see these errors displayed?

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Yes, a white screen generally means that there was a fatal error (like a sytax error). By turning on display errors, it should display the errors to the screen. You could also check your Apache error log for error messages.

The standard install of Elgg should not be creating any WSODs. On what pages are you getting these? Step by step instructions would be useful.

I have been getting alot of these and the error is almost always with canEdit() 

Its a plugin i have added that is causing it but i have not found it yet.

Fatal error: Call to a member function canEdit() on a non-object in /var/www/duuit/html/engine/lib/entities.php on line 1481

was caused by geolocation plugin. fyi

You should report it to the plugin author.