The message you have entered is too short. ? Err, no it isn't!

[archet1337]

When posting using the quick reply I get this message even when posting several sentences:

"The message you have entered is too short. Please lengthen your message to at least 3 characters."

When I click the back button in the browser and submit again it works. But this is really annoying.

Firebug also show the following error on the showthread page:
Syntax error - vbulletin_quick_reply.js
ass("qr_error_tbody","hidden");vB_Edit...;return false}}var vB_QuickReply=true;

[Lynne]

Create a new style with no parent:

Styles & Templates > Style Manager > Add New Style
Parent Style: No Parent Style
Title: Default vBulletin
Allow User Selection: Yes
Save

Then browse the site using that totally default vbulletin style - do you still have the same problem?

If it is still happening - Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

define('DISABLE_HOOKS', true);

[archet1337]

I found the problem. Was this mod:
https://vborg.vbsupport.ru/showthread.php?threadid=235535

Uploaded the original file and could post like normal again :)