It's a fairly 'generic' error (google the error message without the numbers) only indicating that something causes your server's set memory limit to be exceeded. What triggers it, like in your case one user, isn't as relevant as the cause, which can be one or several memory hogs likely not related to a specific user or CB in general.
In other words, whatever triggers the error can be only the one last drop causing the bucket to run over. But there are probably some other processes using up the majority without you knowing.
Two ways of fixing it:
(1) increase the memory limit (it's effective, but in fact only a "band aid", not reducing the memory usage), or
(2) find and eliminate the memory hogs (means going to the root of the problem).
(1)
Here's a good article
on increasing the memory limit. Though your host might use not cPanel but a different interface, and/or not allow you increasing memory, and/or not let you access php.ini. Then you'll need to contact your host.
(2) To find memory hogs in your site, the usual developer tools are helpful: Joomla Debug (a memory hog itself, though!), browser tools, etc. Even simple things such as reducing the sizes of large pictures can help. You may as well scan your other extensions for memory-hungry processes and operations, and disable them for testing purposes, eventually one by one.
If you searched for "Fatal error: Allowed memory size" here on the forum, you'd get the same plus maybe some more pointers for finding the culprit in your specific case.
krileon
