Please report this error to the mod author.

Error while running event minime::on_player_joined_game (ID 44)
minime/scripts/character.lua:811: {
self = "userdata"
} is not a valid character!
stack traceback:
[C]: in function 'error'
__minime/scripts/character.lua:811: in function 'copy_character'
minime/scripts/player.lua:371: in function 'update_armor'
minime/scripts/player.lua:127: in function 'initialize_player_previews'
minime/scripts/player.lua:604: in function 'init_player'
minime/scripts/player.lua:697: in function 'event_handler'
minime/scripts/events.lua:194: in function <minime/scripts/events.lua:191>
2112.045 Error ServerMultiplayerManager.cpp:91: MultiplayerManager failed: "The mod miniMAXIme: Character scaler and selector (1.1.12) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event minime::on_player_joined_game (ID 44)
minime/scripts/character.lua:811: {
self = "userdata"
} is not a valid character!
stack traceback:
[C]: in function 'error'
__minime/scripts/character.lua:811: in function 'copy_character'
minime/scripts/player.lua:371: in function 'update_armor'
minime/scripts/player.lua:127: in function 'initialize_player_previews'
minime/scripts/player.lua:604: in function 'init_player'
minime/scripts/player.lua:697: in function 'event_handler'
minime/scripts/events.lua:194: in function <minime/scripts/events.lua:191>"
2112.045 Info ServerMultiplayerManager.cpp:794: updateTick(26441618) changing state from(InGame) to(Failed)
2112.059 Quitting: multiplayer error.
2115.248 Info ServerMultiplayerManager.cpp:140: Quitting multiplayer connection.
2115.249 Info ServerMultiplayerManager.cpp:794: updateTick(4294967295) changing state from(Failed) to(Closed)
2115.250 Info HttpSharedState.cpp:54: Downloading https://multiplayer.factorio.com/remove-game/12518039
2115.842 Info UDPSocket.cpp:218: Closing socket
2115.842 Info UDPSocket.cpp:248: Socket closed
2115.842 Info UDPSocket.cpp:218: Closing socket
2116.452 Info UDPSocket.cpp:218: Closing socket
2116.452 Info UDPSocket.cpp:248: Socket closed
2116.457 Goodbye

save file
http://file.plexpt.com/k2se2.zip

?

Hi! Sorry for not getting around to this any sooner. Even with your saved game, I can't reproduce the crash. I've renamed myself to "plexpt" and used "Host multiplayer game" to load your game. But all I see is all players signing off, then player plexpt joins again, but everything goes alright -- no crash for me. Perhaps that's normal for saves from multiplayer? Don't know …

The error message isn't really telling either. Could you change line 811 of minime/scripts/character.lua to this:

error(minime.argprint(dst) .. " is not a valid character!")

and try again, with debugging to log file enabled? I'd also like to see the log file as I can't reproduce the crash myself. :-)