I talked to the developer, according to this data it is impossible to determine exactly why this error occurred, it can be assumed that some other plugin tried to rename or renamed the Boss, because this is the first time you see this error (and did not find it in the logs for the previous days, if i understand correctly), aggravating the situation.. if you could track at least some pattern, for example, over the past day you will consistently have 1-2 such errors, I could send you a fix in *.cs, then after a day we could check whether these 1-2 errors per day will disappear or not..
Watch the server logs for a while, look for this line in the logs for the previous day - Failed to call hook 'OnEntityTakeDamage' on plugin 'BossMonster, maybe some kind of system will appear.