Sorry, interview times are currently unavailable. "Internal Candidates"

We are currently having a small issue where a very small number of internal candidates are receiving the message "Sorry, interview times are currently unavailable" when they are trying to book an interview slot.

Its not effecting external candidates at all and its not affecting all internal just a very random small amount.

I have gone through the Job Req's where it works and the ones where it doesnt and they are virtually identical, apart from the actual job. Everything else including interview calendar set up and time slots are virtually mirrors of each other.

I have also looked through "Recruiting Content Library" to see if there is anything unique in the "invite for interview" notification and link but again mirrored to ones that work.