rake test:functionals -> Task not supported by ''

Google turns up little, and nothing with an empty supported-by string.

Apparently I've bent something, but I don't know what. Will someone
please help me get back on track?

If the problem is a source code change rather than an environment
change, have you tried revert to previous source to see if you can
isolate the change that broke this? (if you are using git then git
bisect is designed for this sort of stuff)