Possible a bug with CB Content bot substitutions and other shortcode

I'm trying to ise CB Content Bot plugin to show different content into an article according to usergroups. The CB substitutions work fine until I put inside the substitution the shortcode from the Joomla Quiz plugin to load a quiz from that component:

[cb:if usergroup includes "2"]{quiz 6}[/cb:if]

In this case I got a 503 error.

I've tested another solution using the plugin "Content Filter" ( www.function90.com/docs/content-filter.html ) and in this case I got NO errors, so I thought it could be something related (maybe a bug?) with CB Content Bot plugin.

What could be wrong?
Thanks in advance.

If they're both content plugins then order CB Content Bot first so it can determine if that text needs to be removed entirely or not. My guess is {quiz 6} is returning a string too large for preg_replace (there is a length limit and there's nothing we can do about it), but if CB conditions it before it's parsed then it's just show/hiding literal {quiz 6} which then gets parsed afterwards by its content plugin.