WARNING!
Older versions of this mod for Minecraft 1.8 - 1.12 contain a known security vulnerability. Updated versions for MC 1.10.2 and 1.12.2 are in development - if you play those versions please update all your mods ASAP.
You can read more about the vulnerability here: https://github.com/dogboy21/serializationisbad/
I do not recommend playing very old Minecraft versions on public open servers, you can still safely play in single player.
MDECore is not the only mod affected by this issue and I highly recommend installing the patcher mods below and updating your mods as new versions become available.
If you do want to play multiplayer I recommend installing one of those 2 mods, though I can't guarantee they completely fix the issue - https://github.com/dogboy21/serializationisbad/releases or https://modrinth.com/mod/pipeblocker