@Linkat,
I was speaking to someone the other day and made the suggestion to "always" check out who your neighbours are before you purchase.
I should have taken my own advise 4 years ago:)
Revenge usually comes in "small" ways from a crazy neighbour.. Mine, twisted a metal ornament placed in the garden, because I dared to take a dead cat out of their bin, which their dog killed and bury it... They realised the bin was light..
They didn't realise I saw them chuck it in there.
I can relate to the lock, as I also after that had trouble myself opening my door.
I searched for gum or something, in there but there was nothing. Having said that, it works fine now... So, I've put that down to being paranoid
And, that the "weather" created the problem, either something was stuck in there or something.
If you ever find that again, walk away and get someone else to be there with you to attempt to open it.. That's not me being paranoid rather, better to be safe than sorry, what if it wasn't nature, nor her, rather someone had done it so you had your back turned for ages? Better to be safe than sorry.