There seem to be a couple of problems:
RETURN CODE: 126 means that command invoked cannot execute = Permission problem or command is not an executable. So most likely usr/local/bin is not directly executable and you need to change permissions to allow all users execute that binary or need to create a symbolic link to that binary or need to check that some disabled function of php (disable_functions in php.ini) or some security module does not prevent executing of this script...
File filter/tex/mimetex.linux returns the same code 126 even if the file permissions are 755 so my guess is that the problem is in some security settings (mod_security, disable_functions etc)