Whichever CB page I try to access from the frontend gives me this error:

The website encountered an error while retrieving http://xxxxxx/index.php?option=com_comprofiler&Itemid=53. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this webpage later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

I followed the install info, and checked my server logs but still can't find the problem. I have also used the phpBB3 import plugin which seemed to work fine.

This is almost certainly not a bug in Community Builder, rather a server side issue.
This could be a whole host of things, hard to pinpoint what would be causing the 500 error without more details.

The first thing I would do is rename your Joomla index.php file to something else temporarily and creating a simple index.html file just saying "Hello World" or whatever to verify that it's not a hardware issue or server configuration issue.

The next thing I would do is check your permissions, try chmodding everything to 777 temporarily just to see if it fixes the problem. You may also need to try changing the ownership of the files as well, if you are on a shared hosting environment or don't have root access to your server it might be best to contact your host about this.