In the function onDamageReceived of lindwurm_tail.nut, there is a call to buildPropertiesForBeingHit where it passes the wrong parameters.
Specifically it is on Line 202 local p = this.m.Body.m.Skills.buildPropertiesForBeingHit(_attacker, _skill, _hitInfo.BodyPart);. In this line the third parameter should be _hitInfo, not _hitInfo.BodyPart.
The wrong parameter remains benign in most cases, however, if the lindwurm has a status effect which tries to access the hitInfo, it bugs out the entire skills and makes the tail unkillable.
Solution: Replace _hitInfo.BodyPart in that line with _hitInfo.