I'm running Joomla 1.5 and CB1.2 on a 1and1 Developer pkg.
Got the same error while trying to add a field but solved the problem by adding...
[code:1]AddType x-mapp-php5 .php[/code:1]
to the .htaccess in Joomla's root directory.
That will make PHP5 active. I know that Joomla is written in PHP4, but for some reason PHP5 solves the problem
Don't replace your .htaccess if you're using SEF URLs, just add the line at the beginning, otherwise you will get "Not Found 400 errors" every time you click a link.
Also .... I've added this:
[code:1]memory_limit = 40M[/code:1]
in the hosting's root directory.
these two directories may be the same if Joomla is installed in the root of your hosting, but not my case.