Iv'e noticed this more so with the spawners we have in our "raiding" areas. I'm not sure what happened but some of the spawners we had set up disappeared at some point and when I try to reconfigure a spawner it simply says "You must enter a valid Mythic Mob. That one was not found!".
After seeing this I went straight to my mobs folder to see if the mob was still there. It was. Here is the Mob within its config file.
Plugin not reading mobs file.
After seeing this I went straight to my mobs folder to see if the mob was still there. It was. Here is the Mob within its config file.