Since the Winter's Feast event finished, Klaus (and his sack) is supposed to spawn only in winter (and only once per winter, if I'm right?). In my world however, his sack happened to appear at its regular lockation in the middle of spring. I could open it and fight Klaus as normal.
Also the previous winter was different from usual as the loot stash spawned just on the 3rd day of winter, after the snow fell and covered the lands. (Is it supposed to be like that?) I killed Klaus, but his stash respawned in the same winter, in less than 10 days. After this winter it spawned again in spring. Everything was fine every time I fought him, no other strange thing happened.
My world has the event set to default.
This was the first winter when this happened, I had only one chance every year before.
A regular world, kill Klaus asap, his sack might respawn in the same winter, and/or after winter.