Fatal Error.

Hahhh ;-P LOLZ ;-X
You've either got a bad PlugIn or corrupt language files.
I created this error by accident when testing some PlugIn code  here on my XAMPP !
-- My own code - so on that case I knew exactly what that code was ;-)
If the cause is a PlugIn -- then that is sending 'headers' (some text) before output in expected - and before Elgg expects any output - before the Elgg forward)) is done.
If language file - could be *any corruption - junk chars (maybe hidden by the encoding)
Try reloading all your language files *and disable all non-core PlugIns (maybe even all PlugIns) then test.

Ok i disabled all my plug in and tried to activate one it it was fine.... al though I have to find which one it is causing it... and I have a lot... damnit :p

Oh found them :P

TY!

;-) What was causing the problem ??

Yes, if you found out please share so anyone else facing the same issue knows where to look.

I had the same issue and found it was the mobile 1.8  plugin. I am runing elgg 1.8 version. Hope this info helps someone else.

The same happened to me when testing the Facebook theme (not the latest version) on Elgg 1.8.1beta. It might be caused by themes/plugins not being completely up-to-date with Elgg 1.8.1.

Well -- pity that Mister Nick don't wanna come back and post details of his experience and the exact cause of his error.. esp after I directed him to the solution ! Says much about his 'community spirit' for helping other newbies like him here ! ;-)

@II:

It is not quite 'out-of-date' PlugIns -- "Halting execution for security.." is the reason - The core validations detect that situation where there is a possible security threat and therefore 'halts' ;-) In my case I knew the exact code I had then just added while coding some 1.8 PlugIn here - nothing out of date in my code ;-P

The error made me study the code I had to figure what it was doing that was considered  security threat and... yes ! potenial threat...

I guess that others' PlugIns might end up (unknowingly) having that code style that triggers the 'error'. It's a simple piece of code that opens up a small security hole - but Elgg Core traps that.

I got the same error when I installed 'LiangLee_metatags' - I'd to delete that manually since i was not able to logon.