As mellowhost uses suphp instead of running php as apache module, thats why 500 error occurs a lot from clientside. In most of the cases 3 issues are concerned regarding the 500 error.
1. Using php values and flags in .htaccess
2. Using a php file which has 777 or 666 permission
3. Putting wrong code in .htaccess (Other than php values)
To solve the 1st problem, use a php.ini file to put all the php values and flags. Previously to enable register_globals htaccess was used and it was defined like this under .htaccess:
php_flag register_globals = On
In mellowhost, to enable register_globals just put the following code under a php.ini file created in public_html:
register_globals = On
To solve the 2nd problem, make sure you do not have a php file 777 or 666 permission. If you are unsure which file is 777 or 666 moded then just contact our support center to take care of it, they will make all the files under public_html to 755, and this should solve the problem.
To solve the 3rd problem, just make sure what you are writing in .htaccess are correct. In most of the cases, this is included in 3rd party documentation so its always recommended to contact the script owner / writer to solve such cases if you are unsure about the error.