By any chance did you change the core code. If yes, which onces did you change?

No - sounds like a bad plugin or a bad translation file. Make sure you are not using an editor that inserts BOMs into the text. (This is byte order mark)

I don't made changes in any code, I am translating all elgg in my language and installing plugins. I found problem - it was badwords plugin and now all is working just fine.

No this problem is again. I was activating custom indexes plugin and this error shows again, but there wasn't all that code - shows only -

 Redirect could not be issued due to headers already being sent. Halting execution for security. Search http://docs.elgg.org/ for more information.

and on the top of fatal error page shows -

 Mac OS X 2��ATTR��M�Mcom.apple.quarantineq/0001;4e6e5452;Safari;B1C3F823-540C-459A-8E0E-65FF84B46638|com.apple.Safari 

Remove all the files from your project except the engine/settings.php and try to rplace them with a new download package.

Here is a post on the Mac discussion forums about this: https://discussions.apple.com/thread/3145071?start=0&tstart=0

I got this error today but was able to fix it.  I had Actived the core plugin, "Custom Index 1.8" by accident and without realising.

As soon as I deactivated it the site was functional again :)

I'm running into this error page when I try run upgrade.php.

Could this be bacause I'm manually added Poll object subtype entry to left column of 3c riverdasboard language file. And what is the workaround?

//Polls
    'river_activity_3C:poll' => 'Polls',

@Roman: most likely this error is caused by you modifying the language file. If you haven't made a syntax mistake when adding the additional language string, the error is not due to the entry itself but due to the way you saved the file. You need to save the file either as plain ANSI file or in case of any special characters included in "UTF-8 without BOM" format. Best would be to save it in "UTF-8 without BOM" format. Then you don't need to worry about any special characters being included.

opened and saved it as ansi ASC-II file, still getting error page on upgrade:

Fatal Error.

Redirect could not be issued due to headers already being sent. Halting execution for security. Search http://docs.elgg.org/ for more information.

SecurityException Object
(
[message:protected] => Redirect could not be issued due to headers already being sent. Halting execution for security. Search http://docs.elgg.org/ for more information.
[string:private] => exception 'SecurityException' with message 'Redirect could not be issued due to headers already being sent. Halting execution for security. Search http://docs.elgg.org/ for more information.' in /home/romancos/public_html/engine/lib/elgglib.php:151
Stack trace:
#0 /home/romancos/public_html/index.php(22): forward('activity')
#1 {main}
[code:protected] => 0
[file:protected] => /home/romancos/public_html/engine/lib/elgglib.php
[line:protected] => 151
[trace:private] => Array
(
[0] => Array
(
[file] => /home/romancos/public_html/index.php
[line] => 22
[function] => forward
[args] => Array
(
[0] => activity
)

)

)

)

 SOLVED!!!!

 I just figured out something. Error page doesnt show when I turn debug mode on prior to running upgrade.php Is this intentional? It makes sense if you need debug mode on for running upgrade.