Yes, the short fix is to remove "unknown" from the variable
private static $teachernotallow
in the casstring class: https://github.com/maths/moodle-qtype_stack/blob/master/stack/cas/casstring.class.php
I'm conservative about what has been included, particularly with functions which have global scope within maxima. The "assume" database of functions does precisely this for example. Unknown is both a maxima function, and a return value (confusingly!) But I see no reason to avoid using it, so I'll commit come changes which enable this to be available in future versions.
If you get this question working *reliably* please let me know!