Hi there, First, I love this mod.
We've just started receiving the below error.
As far as I can tell no one had clicked it or opened it at the time.
And everything was running fine until a third person joined.
Let me know if there's any other info I can help with.

Installed mods:
factoryplanner_2.0.16
flib_0.16.0
fox-todo_2.1.0
helmod_2.0.15
notepad_1.0.4
quick-bar-switcher_1.0.0
TaskList_0.5.0
welcomemotdbox_2.0.0

We are playing the PVP Scenario

Error:
17577.374 Error MainLoop.cpp:1435: Exception at tick 127047577: The mod Fox TODO (2.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event fox-todo::on_surface_deleted (ID 66)
fox-todo/util.lua:10: attempt to index local 'tag' (a boolean value)
stack traceback:
fox-todo/util.lua:10: in function 'is_valid_tag'
fox-todo/gui.lua:36: in function 'should_show_tag'
fox-todo/gui.lua:195: in function 'render_todo_gui_player'
fox-todo/gui.lua:225: in function 'render_todo_gui_force'
fox-todo/tags.lua:74: in function <fox-todo/tags.lua:49>
17577.375 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Fox TODO (2.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event fox-todo::on_surface_deleted (ID 66)
fox-todo/util.lua:10: attempt to index local 'tag' (a boolean value)
stack traceback:
fox-todo/util.lua:10: in function 'is_valid_tag'
fox-todo/gui.lua:36: in function 'should_show_tag'
fox-todo/gui.lua:195: in function 'render_todo_gui_player'
fox-todo/gui.lua:225: in function 'render_todo_gui_force'
fox-todo/tags.lua:74: in function <fox-todo/tags.lua:49>"
17577.375 Info ServerMultiplayerManager.cpp:806: updateTick(127047577) changing state from(InGame) to(Failed)
17577.375 Quitting: multiplayer error.
17577.375 Info GlobalContext.cpp:760: Deleting active scenario.
17579.435 Info ServerMultiplayerManager.cpp:146: Quitting multiplayer connection.
17579.435 Info ServerMultiplayerManager.cpp:806: updateTick(18446744073709551615) changing state from(Failed) to(Closed)
17579.435 Info ServerMultiplayerManager.cpp:166: Deleting active scenario (global = true).
17579.436 Info HttpSharedState.cpp:55: Downloading https://multiplayer.factorio.com/remove-game/20123401
17580.279 Info UDPSocket.cpp:233: Closing socket
17580.279 Info UDPSocket.cpp:263: Socket closed
17580.280 Goodbye

256.715 Error MainLoop.cpp:1435: Exception at tick 127046907: The mod Fox TODO (2.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event fox-todo::on_surface_deleted (ID 66)
fox-todo/util.lua:10: attempt to index local 'tag' (a boolean value)
stack traceback:
fox-todo/util.lua:10: in function 'is_valid_tag'
fox-todo/gui.lua:36: in function 'should_show_tag'
fox-todo/gui.lua:195: in function 'render_todo_gui_player'
fox-todo/gui.lua:225: in function 'render_todo_gui_force'
fox-todo/tags.lua:74: in function <fox-todo/tags.lua:49>
256.715 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Fox TODO (2.1.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event fox-todo::on_surface_deleted (ID 66)
fox-todo/util.lua:10: attempt to index local 'tag' (a boolean value)
stack traceback:
fox-todo/util.lua:10: in function 'is_valid_tag'
fox-todo/gui.lua:36: in function 'should_show_tag'
fox-todo/gui.lua:195: in function 'render_todo_gui_player'
fox-todo/gui.lua:225: in function 'render_todo_gui_force'
fox-todo/tags.lua:74: in function <fox-todo/tags.lua:49>"
256.715 Info ServerMultiplayerManager.cpp:806: updateTick(127046907) changing state from(InGame) to(Failed)
256.715 Quitting: multiplayer error.
256.716 Info GlobalContext.cpp:760: Deleting active scenario.
258.412 Info ServerMultiplayerManager.cpp:146: Quitting multiplayer connection.
258.412 Info ServerMultiplayerManager.cpp:806: updateTick(18446744073709551615) changing state from(Failed) to(Closed)
258.412 Info ServerMultiplayerManager.cpp:166: Deleting active scenario (global = true).
258.415 Info HttpSharedState.cpp:55: Downloading https://multiplayer.factorio.com/remove-game/20125422
259.267 Info UDPSocket.cpp:233: Closing socket
259.267 Info UDPSocket.cpp:263: Socket closed
259.268 Goodbye

Also just occurred to me that one of the map tags I'd added to Fox TODO was of another players ship.
They're in a different team and their ship was getting destroyed.
He had just reconnected to the server as this was happening and then we got the error.

I have published 2.1.1 just now which should hopefully fix this crash.

This crash would occur basically whenever a surface with TODO tags on it would be deleted. Let me know if you still experience any issues.

//EDIT2: No data changes should actually be necessary. The mod should automatically refresh tags on the new version. 2.1.2 and 2.1.3 are no code changes, just me fixing up the changelog I initially forgot to publish

Wow, amazing that you've already looked at this and potentially fixed it!
Thank you so much!
I'll wait until everyone's off the server before re-enabling the mod and test it.
I'll let you know if there's any issues.
Thanks again, you rock. :-)