Passenger side rear door won't open from outside

My mum's Citigo's passenger side rear door is having problems lately. It won't open via the outside door handle. Inside door handle still works/opens the door but it's much stiffer than all the other inside handles. Any idea what the likely issue(s) might be?

Its 3-year new car warranty is up in about 3 weeks - is this something that would be covered under that warranty so I could just ask her to take it to the dealer for a proper look/fix rather than any potential bodge by me.

Thanks for looking/helping. Cheers.