Regarding your warning (if it still exists). This sounds a bit like too much (useless) bones are assigned with (usually almost 0.0) weight to vertices. If you do that “Clean” step make sure you don’t limit to one group (e.g. if you selected only one bone) but change the pulldown to all groups (in the left “T” sidebar)… and probably even set some limit 0.00001 or so. You can select single vertices in the weightpaint mode and you could see the assigned bone/vertex_group names in the “N” sidebar afterwards. For example at some vertices near the ellbow you would see some values for upperarm and lowerarm bones. For example 0.5 upperarm 0.499999 lowerarm and 0.000001 some foot_l (which is **** at the hand as the hand does not require the foot to tell him how he should move the vertices around) or some other bone somewhere far away or from the right side instead the left (caused via X-mirror painting earlier). It sounds like you have for some reason 8 bones assigned to one vertex. “Clean” in Blenders weightpainting mode should remove all such wrong assignments below the specified weight. So if you set some limit it would left for example 0.5 upperarm and 0.499999 lowerarm. And “normalize all” should set afterwards the missing 0.000001 to one of those two bones. However it seems even UE4 made some kind of clean now as it seems everthing below 0.005 was dropped anyway (I’m not sure if this is the lowest weight in UE4 or just the lowest weight after there are too much bones pulling on one single vertex).
Regarding that Death_Ironsights… I think this is an anim bug. Probably many don’t care because you could get better “animation” via physics ragdoll (instead of keyframed anim) to “die”.