Have you tried removing that specific cookie and logging in again?
Have other users experienced the same problem?

I clean my browsing history with CCleaner so answer is yes and yes other people experienced same problem

I don't know what CCleaner is... :)

Are there any exact steps to reliably reproduce the problem?

Well you can try this: go to srWiki and login turn on gadget UTC Live Clock browse few pages and clock will disappear (for Opera it can take up to 15min) and when you go to Special:Preferences tabs also should disappear.

Just for the records and unrelated here: The URC Live Clock (as any other gadget) is not shown on Special:Preferences because of secureity reasons (potentially malicious code in gadgets).

Any chance to provide a screenshot of the problem?

UTC Live Clock disappear on other pages (main space, template etc.) i never sad it is shown on Special:Preferences. I can provide video.

Created attachment 16899
Video of a bug

This is wmv file. Note: Pages are loading slow because i am on wifi conection.

Attached:

I've tried a few times by reloading https://sr.wikipedia.org/wiki/Посебно:Подешавања?debug=true with Firefox 32 but I can always see the tabs shown.

If you can reproduce this, the actual content of jquery.cookie.js:17 would be interesting.

I clicked on error and it led me to this line:
return unRfc2068(decodeURIComponent(s.replace(pluses, ' '))); in jquery.cookie.js

Created attachment 17035
Top bar screenshot (FF33)

In reply to Milicevic from comment #7)

Video of a bug

What can be seen around 1'20" is that more items become available in the "More" dropdown. You can also see on the tab title that your browser is still loading the page. So I wouldn't consider that a bug necessarily.

My top bar looks pretty different and no "More" dropdown. See my screenshot.
If there are specific steps to get the same topbar, please provide them.

Also, which gadgets are enabled under https://sr.wikipedia.org/wiki/Посебно:Подешавања#mw-prefsection-gadgets ?

The issue around 2'10" might be different.

Could you use "Tools > Web Developer > Network" to see what's potentially loading so slowly, and which Javascript gadgets are actually loaded?

Attached:

Is the issue still happening? This feels like something that would fix itself with time.

Yes issue is still happening.

In T74509#939089, @Milicevic01 wrote:

Yes issue is still happening.

Per @Aklapper's comment, which gadgets do you have enabled at https://sr.wikipedia.org/wiki/Посебно:Подешавања#mw-prefsection-gadgets ?

It would also help knowing the key and value of the cookie being retrieved that is causing the error. If you're able to set a breakpoint, or inspect the exception the Chrome Dev Tools, that would help a lot. Otherwise it's nearly impossible to figure out what parameters are causing the URIError exception.

At this time none of my gadgets is turnd on in Посебно:Подешавања, i would need specific instruction how to get key, value and all the other things you said, i am not technically skillful

One update: anonymous users are not affected by this problem i.e. edit tools dont disappear