Fatal error related with collapsible containers

First of all I don't see collapsible containers functionality at all on my Drupal 5.10 + Advanced forum alpha-12 + blue_lagoon theme. may be it is supported only for drupal 6, then sorry.

And the big problem is
Firebug reports:

Fatal error: Call to undefined function drupal_json() in /home/mysite/htdocs/sites/all/modules/advanced_forum/advanced_forum.module
on line 1099

where the line 1099 looks like:
print drupal_json(array('tids' => $initial_state));

If I am not mistaking , drupal 5 doesn't have such function (drupal_json) may be this is the reason why it is happening.

But the bad thing is that I even can not switch off collapsible containers in the AF settings in the section "Default collapsed containers:" when I uncheck all containers, and press "save" - the first container always gets checked. It gets, however unchecked in the case if some other container set to be collapsible by default - let say if i leave just last container checked and others unchecked then these settings will be saved correctly and first container will not get checked like before. In any case the error reported by firebug occurs.

on the demo site of advanced forum, link to which is given on the project page - it also happening, the same firebug report.

I will greatly appreciate any help or advise. Thank you.