Can't Add External Collaborator

I am trying to invite our board treasurer as an external collaborator within our Enterprise account, but system is telling me it won't work. I have done this plenty of times before with external collaborators with all kinds of email addresses. I do see where our folder settings say "restrict collaboration to within the company" but it is greyed out so I cannot change it, and also, I already have other external collaborators on this same folder so how was I able to invite them but not this new one? Please help.