When you reschedule appointments in a roombox you can get this error:
Your meeting request was declined.
It didn't specify an end date. The end date must be before x/x/xxxx.
To fix this start the Exchange Management Shell and set this commands:
get-CalendarProcessing * |fl |findstr Roombox
set-CalendarProcessing "Roommailbox" -EnforceSchedulingHorizon $false
set-CalendarProcessing "Roommailbox" -BookingWindowInDays 1080
After this cmdlet you can schedule the appointments more than the default 180 days.