First bit of new functionality work i’ve done in ModX for a longtime and I came unstuck by not being able to create any new template variables, due the the fact that ModX thought I had invalid characters in the variable title.
After a bit of googling it appears to be a PHP version problem, with PHP 5.5.10 not playing nicely with the string checks in ModX. To get round the issue I simply switched PHP versions in MAMP, added the variables, then switched back and carried on as normal.
Not a fix as such, but enough to get the job done.