Strange bug

Well, I've got a strange bug. Quite convenient, though. On 1 of my characters I get no damage from backstabs. I still get hit by mobs, receive damage from other players normal attacks, but I get totally no damage from backstabs. Didn't really test ripostes, but I feel kinda frustrated to use this character any more. Seems so unfair.
Luckily, it is a cosplay build, and it is SL 101 already.

Wow that's pretty interesting..... you must be a nightmare for bs fishers haha. Any idea how or when this started happening?

Well, I was just messing around, cosplaying as Leeroy. Beated Manus, accidentally levelled to SL 101, then moved to the Forest... I just noticed yesterday that 1 guy sometimes could not damage me with his Uchigatana hits, even though he clearly hit me (no lag). The bleed meter was filling, but he didn't hit me enough to let bleed trigger - just waved to me and ran off the cliff. Then another guy came by with a rapier and was quite good at damaging me. Then we met up with Xero, he tried to swing at me with Fire +10 Mura, and actually did some damage and when he BSd me - I got totally no damage. Afterwards I gave it a try to another guy with pure physical dmg on his Zwei - no damage. And another guy with lightning Zwei sliced me pretty good. However, just realized, he riposted me with Lightning Zwei for some damage...
Frustrating. Really. Feels like I'm invulnerable to some type of physical damage? %) I have like 400 of phys resist.

Ed: By the way - all the mobs hit me quite well.

I did around 400 damage with that backstab. At least thats what it said on my screen.

Errr... I didn't see any damage on my health bar %) When you hit me a regular way - it leeched like 15-20% of the bar, but on BS - the bar kept on the same level...