Incompatible with https://mods.factorio.com/mod/blueprint-sandboxes
Just report. Maybe make it compatible or add the incompatible list :D

44.267 Script @blueprint-sandboxes/scripts/force.lua:47: Force.Init: Main Force
44.268 Error MainLoop.cpp:1429: Exception at tick 0: 模组「Oarc 多人游戏 [Oarc Multiplayer Spawn] (2.1.5)」引发了无法恢复的错误。
请向模组作者反馈此错误。

Error while running event oarc-mod::on_player_created (ID 27)
模组「蓝图沙盒 (2.1.5)」引发了无法恢复的错误。
请向模组作者反馈此错误。

Error while running event blueprint-sandboxes::on_player_changed_force (ID 60)
blueprint-sandboxes/scripts/sandbox.lua:357: attempt to index local 'playerData' (a nil value)
stack traceback:
blueprint-sandboxes/scripts/sandbox.lua:357: in function 'OnPlayerForceChanged'
blueprint-sandboxes/control.lua:115: in function <blueprint-sandboxes/control.lua:112>
stack traceback:
[C]: in function 'newindex'
__oarc-mod/lib/separate_spawns.lua:183: in function 'SeparateSpawnsInitPlayer'
oarc-mod/control.lua:105: in function <oarc-mod/control.lua:104>

I can't reproduce this. I added both mods and started a new game and it seems to load for me? I can spawn normally and then go into the blueprint mode too.
Can you tell me what steps I need to do to see this error?

I tested and figured out what's going on.
If the factorissimo 3 mod is here https://mods.factorio.com/mod/factorissimo-2-notnotmelon, then
the blueprint mod https://mods.factorio.com/mod/blueprint-sandboxes cannot be used, vice versa.
It's very weird. It means the people have to choose the one of them if they used Oarc Multiplayer Spawn.

Though the further test shows the factorissimo 3 is not the only one will cause the crash with blueprint sandboxes...
However, removing the blueprint sandboxes can be the most compatible for other mods. So weird.

1. The mod EdiorExtensions https://mods.factorio.com/mod/EditorExtensions.
2. The mod Blueprint Sandboxes https://mods.factorio.com/mod/blueprint-sandboxes.
3. The mod Factorissimo 3 https://mods.factorio.com/mod/factorissimo-2-notnotmelon.

OK. Lucky that I have the answer now. There are three mods. They will cause the crash with Oarc Multiplayer Spawn when:
1+2 or 2+3

Haha sounds fun to test... Thanks, I'll look into this tomorrow when I get time and see if I can solve it on my side with a simple dependency or something.

If you can, please give the new version a try and see if it works? V2.1.6

Yes, indeed.

The Blueprint Sandboxes now seems like working well with Oarc Multiplayer Spawn in existing save. Good Job :D