@DhrupDeScoop: I've already deleted that part, because i found that code somewhere in the net, I've been trying everything, you know. I can't add here the whole code, because it makes no sense, the bug is here. It can be something with php file add, it seems like widget is not created, the object is empty. But i haven't changed there anything!

@rjcalifornia: I dno not really remember, but if so, it was very little changes, but it doesn't matter - i have the same files, the same site on the otehr server and there it works perfectly. I've been checking every plugin, turning it off and on.Maybe it is something with server?

Widgets 'ADD" is driven by jQuery - so when click on widget to 'add' -- the jQuery JS shud trigger to do on-screen 'add' & shuffle-shuffle and 'added' to the active widgets in the main widgets area below the widgets list. If this not works - issue should be with the jQuery/JS for the 'Add'

Not really, i've just found out that it must be the problem with some rewrite rules in .htaccess. I've had repaired other problem with my server (my css formating was missing on the new server, so i changed cache rewrite rule: "RewriteRule cache\/(.*)$ engine/handlers/cache_handler.php?request=$1&%{QUERY_STRING} [R]") and now, when i'm trying to add widget it shows me a message:

"Form is missing __token or __ts fields";

need to find out how to give ts and action token to that action or  find out what is wrong with my rewrite rule:    

RewriteRule ^action\/([A-Za-z0-9\_\-\/]+)$ engine/handlers/action_handler.php?action=$1&%{QUERY_STRING} [R]

If someone would help me, i would really appreciate it!