I also tried my to run my Yii application on a free hoster (http://www.bplaced.net). It work’s fine but I my case I have to disable one option in the .htaccess file cause the hoster doesn’t support such feature (empty page was displayed). Don’t know exactly the problem but I disabled everything in .htaccess and tried it (working), add one disabled line, try it again and so on until I find the line which cause the problem.
In your case the function debug_backtrace is not available but it is used in CErrorHandler. What you could try is to remove debug_backtrace calls in every core file but I would not do it because than you have modified core file(s) which is not recommended.
So the only workaround for you in my opinion is to switch to a different free hoster like bplaced.
You have to disable debugging and maybe also tracing in the index.php file located in protected directory. Look at manual/cookbook/tutorials how to enable/disable debugging if you do not know how to do it.