Something similar happened to me and it's happening for a while now. I can't connect to Bancho because it says my osu! is "too old". (I just re-installed it again though, due to similar issues in the past where it can't update.)
It updates by itself (I can watch it updating through options) but then once it reaches a percentage (for example, 71%), it goes back to a percentage lower than it (for example, it reached 71% but then it goes back to 69% and continues going up and down). It never reaches 100%. After all the waiting, it tells suddenly me to check my internet connection (like the first post in this thread).