This question about Using an extension: Answered
Cheap fix for permission issues
Problem:
When trying to expand variables you may encounter a permissions issue with the topic and become unable to save the new expanded version of the topic.
Solution:
The issue might be because the previous topic write sets access control on the topic (if specified in the template) which prevents plugin from writing to the topic again.
A dirty solution is to find this snippet of code in
NewUserPlugin.pm
$errorMsg = Foswiki::Func::saveTopicText($usersWeb, $wikiName, $text);
and change it to this
$errorMsg = Foswiki::Func::saveTopicText($usersWeb, $wikiName, $text, 1);
the addition of the 1 tells the saveTopicText function to not check permissions when writing.