I have experienced some similar (or exactly same) strange behaviours after 4.50. We share an internet connection (ADSL) with my twin on one desktop and one laptop and when I send a message to my brother, he mostly doesn't receive it. I don't know why. Sometimes, very lately, he receives them (or some of them or only the first one of them). Both computers have Apple Live Messenger enabled (not sure which skin my brother is using though but it must be ALM). I don't remember whether I sent a message when I'm not using any skin.
But to other contacts out of our LAN, my messages are sent successfully, or I think they are successful because all of the time they reply
warmth, I don't know if you are testing this on a LAN but a LAN could be the cause of this bug, but it is still a bug of 4.50
imho.
Since this issue has been posted, I'll continue testing this as soon as I return home and try to post again.
Edit: XP / 8.5 / 4.50 / ALM skin
/ 
