File bb_func_vthread.php, line 239: $cols[4]='';
It means, that poster_ip cannot be used in parseMessage() function. Why? Any reason?
I ask, because won't make any fatal mistake. This line looks useless and that's suspicious:-)

This value can be used in parseMessage() for sure (as $GLOBALS['cols'][4]); but make sure you are logged as admin or moderator to see this value, and most probably you need to put the proper condition in parseMessage() as well.

In bb_func_vthread.php, follow if then else condition which specifies this value. It appears untouched only in one place.

Yes, I know that I can use $GLOBALS['cols'][4] in parseMessage() as admin or moderator, but I want to use poster_ip as internal value to determine some specific behaviour - the same for each registered member. So question is, NULL value is set for non admin and non moderators because of safety? If I delete line 239, no following problems are obvious?

Yes, it's (obviously) nulled because of safety.
Try to comment this line and let us know the results :) I don't suppose obvious problems at the moment because of that.

I tried and it looked OK with my setting.