The minibus must have 9 or less seats otherwise it should not have a rear side door on the off side. If the nearside door was standard it should be operational especialy if you can not get out though the back doors, I do not know if this is illegal to drive, but will probable fail an MOT .
If it only has two rows of seats it may be OK as the old double cabs only had one door at the back, on the nearside and no windows in them to be exempt from car tax.
If you are the driver I would tell your boss you are not driving it with anybody in the back untill it is fixed, would be better if the offside door was not working.