If a player reaches a level higher than 255, the level shown on the scorboard and at the skills hud-thing starts at 1 once again, one may still use all skillpoints(e.g. the 71 needed to upgrade all skills to their max) and as soon as the "over 255"-points have been used, the number decreases from 255 until all 71 upgrades are done...not a big bug but kinda weird.

255 is the maximum value of a short integer, since it can not hold a larger value, it modulo's the large value and store that result, thusly giving you the value that you see.

Since levels that large rarely come into play, chances are this will stay the way it is...

is it only me or do players lvl up faster (on a server with default exp-settings etc) above lvl 255/300 or so...I partly got 10 lvls on one map, thats kinda weird aswell

That also may be due to the short integer value...

that is probably the point in which "damage done" surpasses "difficulty of leveling". Thus at levvel 255+ your damage bonus is so high that you gain tremendous experience from each hit.

hm the damage done ist calculated by the "skill" skill which is max.30 so after lvl 71 when all skills are at max, there should not be any effect of lvl-upgrades on damage etc

Correct. Though you can get higher upgrade levels on harder difficulties because your starting values are lower. You still end up with the same final result...

Here's what I'm saying:
lvl 5 x, y = 2x
lvl 50 x, y = 2/3x
lvl 200 x, y = 1x
lvl 250 x, y = 1/2x

where x is the damage done and y is the difficulty of leveling.
Thus at level 250+ the damage done surpasses the difficult of leveling, allowing you to level faster because you have broken the barrier.

For those of you who are familiar with Newtonian physics, this effect is comparable to terminal velocity.