MyBB Community Forums

I am administrating 3 different MyBB forums and I am having functional problems with installed themes on two boards. On both forums is installed the latest version of MyBB (1.12) and I can install themes on boards but on first board when I import some new theme users cannot choose that theme in their UserCP (I cannot do that also although I am administrator) and in theme settings stands that anybody is able to use it.
I have set that theme to be default but besides the first described problem there is the second one: No one can post replies or threads and when I try to do that, I get some warning that there is a problem with some code and that's why reply or thread cannot be posted. The same warning I get when I try to change a theme from User CP.

In the other board I can change themes from User CP, but I cannot post replies and threads from the same reason as I have described in previous paragraph.

But on the 3th board where I have kept the version 1.10 (didn't do any updates from then) I can change and use themes normaly, and everything is functioning as it should, so I came up to conclusion that something's wrong on relation new themes and 1.12 MyBB version.

Of course, under default theme everything is normal.

Any ideas to solve this problem so I can use some other themes on my boards?

If you're getting any error messages please post the exact message. Also please post your board URLs and the plugins/modifications you have applied on your boards.

Message in described problems is:

Quote:Authorisation code mismatch. Are you accessing this function correctly? Please go back and try again.

I am getting this message as a board's Administrator, but my user have also reported the same problem with new theme when they try to post replies or threads.
Only plugin in Plugin Manager is default one: Hello World! and it's not activated.
First board's URL is www.zorankom.com

Second board's URL is www.ourlevel.org
Here I have deleted all additional themes cause I couldn't use those themes (I could change themes in UCP, but couldn`t post replies or threads getting the same message like in first board.
Default theme in this board is Webmaster theme and it was set when MyBB version was 1.1.10 (regulary updated so now it runs on 1.1.12 version), and the same theme won't work in first board when I install it.
But when I install on second board for example Black&Blue theme, I can choose and change style from UCP, but cannot post threads or replies.

In this board there are many plugins installed (some are not activated, but you will see for yourself which ones):

List of plugins :

Board Message
Allows administrators to add a global message to the forums header from the settings administration. 1.0.1 Musicalmidget (not activated)
BP No Quote in Quote
Clean up your nested Quotes in newreply page also in Quick Quote box. This plugin make your post to carry just LAST quote and not all of them.So most externall Quote remains and others will clean. 0.3 Bojan P (not activated)
Disable Who's Online for Non-staff
A simple plugin to disable the Who's Online page for non-staff users but still allowing them to view the Who's Online list on the index. 1.0 MyBB Group (activated)
Douple Post Auto Merger
If a user makes a double post (one post right after the other in the same thread by the same author) it will merge the two together as if the user had only made 1 post. 1.0.2 Tikitiki (activated)
Easy Refer
Creates an invitation form which adds the members referral code. Also shows the users referrals. 1.0 Jesse Labrocca (activated)
Forum Warning System
Send your members warnings if they don't follow the rules. 1.1.2 Smethead (activated)
Image resizer
Resizes an images if the width is larger than a maximum width given through the settings panel. 1.1 Cipher (activated)
MYPS
My Points System 1.22 Jesse Labrocca (activated)
Inline thread popup
Shows an inline popup with some of the message of an thread in the forumdisplay page. 2.0.3 CraKteR (activated)
Quotes
An Quotes Plugin. 1.2.4 CraKteR (activated)
Rules page
Adds a page which contains forum rules. 1.0 Chris Boulton (activated)
Side Boxes!
Additional boxes on the right of the forums in index page. 2.0 Zaher1988 (not activated)
Trash Can Forum
Deleted Threads Will Automatically be moved to this forum 1.4 Nickman and CraKteR (activated)
Unread PM Notification
Informs users with a banner at the top when they have any unread messages. 1.0.2 destroyer (activated)
User Agent
Shows icons and stats informing about poster's operating system and browser. Do not forget about $post['icon_os'] and $post['icon_browser'] in your templates. 1.2 bryn (activated)
viewe post
display post 0.8.0 Arash_j13 (activated)

"Authorisation Code Mismatch" and how to fix it Search

Ok. Thanks for a Help,I mean link , but as I can see the instructions on Wiki are about upgrading script, which is the case with my second board, but the first is fresh installed 1.1.12 version.
Anyway I'll try to find solution on that thread...If not, here I am again .