I haven't use Joomla in years, and never used the Kunena module, but just glancing at that Debug code, it looks like it was spit out because of a Database error.
My guess is that this falls back to the UTF-8 encoding issue with the database, since it seems from all of your posts that it happens when using those characters. I can't debug it more without access to the server error_log files, but hey. 
Anyway, the reason I think it's a Database thing, is the output of the variable dump.
| Code:  | 
  
$0 " 
key value 
$file "/var/www/vhosts/dokuga.com/httpdocs/components/com_kunena/template/dokuga/post.php" 
$line "223" 
$function "trigger_dberror" 
[args] Nested Array 
" 
  | 
 
The function being called is trigger_dberror(), in the posting of the, uhh... post. The entire thing, IIRC, is just a trace of how the error was received, with the newest entry at $0 and the oldest at the higher number.