Yesterday, I accepted an offer for an PhD. I emailed the other universities that I had applied, informing them that I made my decision.
Today, I received an email from one of the potential supervisors (not from the uni I got accepted), probably sent to me by mistake, saying: "Thank God! Problem solved!"
I believe I made the right choice and chose uni A, because if I am considered as a problem, before they even met me....I can't imagine what it would be during the research!!!!

I doubt that means they were considering you to be a problem - why would they consider offering you a place if that were the case. Perhaps they were struggling to choose between you and an equally able candidate?

Yeah, I agree with HazyJane!

There is no telling what the reason could be for this ... it could be for other reasons, with funding being one of the prime candidates. It could be internal politics and not a reflection on you at all.

If feeling evil, ya could always respond to the mail, asking whether or not it was it meant for you. Otherwise, just treat it as you would treat a PFO and move on to the opportunity given to you from the other college. Good luck to you in your research.

I am self-funded. So there was not a matter of funding. In fact they liked the project I proposed, the fact that I already had papers in journals and conferences!!!!

I find the comment quite unprofessional, and it might almost be read as him giving you the proverbial two fingers (the fact he didn't explain his comment makes me think this). I'd probably write something back on the lines of...'I didn't know you cared so much (or so little)'.

While we are lowly PhD students, it doesn't mean we have to put up with being talked to like shit.

i would ignore it, don't send anything back, you may find you need support/help later on (maybe post PhD)from this person and being rude is just as unprofessional and may cause you more harm in the long run, academia is a small pool and everyone knows everyone else (or nearly so)so an adverse comment by this person could affect you more than him.

I agree with Joyce. A comment could haunt you later. At the same time, I wonder if said academic should really get away with a stupid comment like that, in particular on a professional level. So I would send an email saying something along the lines: "I think this type of reply is neither professional nor appropriate".

On a private level I would ask said person to meet me outside the farmhouse at midnight to discuss any issues like real men.

Yes, get back to him, cc. to his line manager and ask the meaning of his curt message.

i think this is being over analyled a bit to much and it could mean anything.

It seems highly unlikely the academic deliberately sent that email, and as you've picked the other uni and don't have to work with them, then Id move on and forget about it.

By the sounds of it, this is probably a "Whoops!" and they hit "Reply to All" or something similar. I would take it as a compliment, it might have been that they had some really dire applicants and were relieved to see you! :D