Alright, I found that PlagueLord did not kill the dummy on recast. It simply summon a useless dummy with same family name to replace it. But i still think it won't caused the crash. The most problematic thing is, when PlagueLord dies, it won't kill the dummy nor cast the useless dummy, but apply an affix "blood_charge_off" that clean the stat. So, that dummy will still try to keep the passive with level 0, and the skill that will trigger on caster die. When you cast the skill again, it will reapply the affix "blood_charge_on" without removing the "blood_charge_off". Will it be necessary to remove the affix for clear stats? I don't know. But I think the best practice is still removing any affix when it's not needed.

More Update

Can't find when does it summon the dummy. Seems like that the dummy won't die even if you turn off the skill. It just changes the level of the dummy skill instead of resummon it. The funniest thing is CD or cast once only do no effect. I guess the dummy keeps resummon itself so CD and cast once can do nothing as every time it's a new dummy. Of course, it's also possible that another dummy keep summon that dummy. I believe the process of keep apply buff and resummon dummy cause overload on multiplayer game and finally crash the game sometime depend on network speed or other stuff. It explains the situation that sometime it crashed and sometime it didn't, and explain that even some classes use toggle skills, they cause no crash. However, I still can't find a way to fix it to prevent the resummon of dummy. I have no idea how the summon system works in Guts even after reading the engineering skill.

I have found a possible fix for classes crashing issues during multiplayer/co-op games when transitioning from one area to another. As posted https://steamcommunity.com/workshop/filedetails/discussion/900049612/1694917906661349705/, some crashing issues are due to toggleable skills. I have tested on 2-3 classes, but mainly with Plaguelord.

After I dig into the code of Plaguelord's Body Boost, which have a visual effect of toggle off and different CD (10s after 1st cast, 3s after 2nd) so it is easy to notice the change, I believe that either the skill being reapplied too frequent, or it last after warp caused the crash. However, after I add "DIES_ON_WARP:1" to both dummy units (I don't understand why there are two dummy for this skill), turn it on and warp in game, the buff is still there. But the unittheme is gone and I can only activate the second stage of that skill but will the first stage of CD (10s). The simplest ways I can think of are either trigger the toggle off via cast on warp (but not such thing), or via cast on dummy die (I can't find either). I can't successfully turn the skill off on warp to have enough information to test whether toggleable skills really caused the crash. Can someone help?

Anyway, I will try the same on Ritual of the Chain later to see whether I did it. In fact, it is possible that the passive of Plaguelord, which work as toggleable but no toggle off function, can cause the crash too.