It is odd as a few other people mentioned the same problem... it sounds a bit like a ping problem, but so far most of those who had similar issues still had quite acceptable pings. Now I for my part rarely ever notice such behaviour in the game when playing, although yesterday I was sure to have hit a certain fortunate archer three times with my mace from horseback without any effect. Also I recall that a while back there was an incident in which I indeed constantly hit people (not from horseback), with the weapons moving through them without doing damage, or javelins and arrows simply ignoring targets and moving through them, but later that day it was all as it should be again.
Now for all I know there is no fix or anything that could really be done about it, as we are not even sure what the cause could be. From what I know about warband there seems to be something like a memory leak issue, so that the servers get clogged up after running for more than two weeks or so, and after not being restarted for a while issues like these become more common. It is a bit difficult to compare the server behaviour right now as FenrirSkogr is the only one that is up, but to my knowlege it has been restarted more or less regularly over the last time.
The odd thing is that these kinds of problems seem to be worse for some than for others, as I for example rarely ever have any complaints like that. It is difficult to explain that with a server issue, and so it might be worth it to investigate a bit if you maybe have some programs running in the background that worsen your game performance. That doesn't even have to show in the fps count, but I noticed that when you for example are currently having a youtube video opened and play the game at the same time, the game will run quite a bit less well than normally. I suspect it has to do with the core-management of warband although I don't really have a deep understanding of it.
But i have heard of a whole register of problems like this, ranging from javelins and arrows missing targets and magically "teleporting back" into them to score a kill, people getting hit through shield and non-shield blocks, weapon blocks showing a different direction for one player than for the rest and similar. Still as far as I know that is not really Vikingr-specific, and although it would be nice to get rid of these problems I don't really know if we can do anything about it in terms of modding itself