Please attach your savegame.

I reproduced this, here is a simple scenario that uses 'CityGrowthLimiter v6'
cgl.zip

The bug is triggered by the script in town.nut during static initialisation:

class Town {
        ...
        paxRequired = GSController.GetSetting("min_transport_pax");
        ...
}

• The script wants to read some script configuration setting during loading of the scenario.
• OpenTTD is not yet finished loading the scenario and actually thinks it still is in the main menu (GM_MENU).
• OpenTTD then tries to use the new-game script configuration, instead of the one from the scenario. (this is already wrong)
• However, this new-game script configuration is not available on dedicated servers, so OpenTTD crashes.

The underlying issue in OpenTTD is actually pretty severe:

• When loading savegames/scenarios, AI and GS are started inside the saveload code:
  • AIPLChunkHandler::Load
  • GSDTChunkHandler::Load
• At this point of time, the savegame is not yet completely loaded, AfterLoad did not run yet, ... so the gamestate is still incomplete or even invalid.
• AI/GS code running at this time get in contact with all kind of invalid data. (static initialisation, script's Load() method)

I think to fix this, loading AI/GS needs to be changed significantly:

• AIPLChunkHandler::Load and GSDTChunkHandler::Load have to store the raw script data in some temporary location.
• Loading and starting AI/GS must wait until the end of AfterLoad, when the gamestate is ready.

I can reproduce this with a custom AI specifically made for this, so it is not just limited to the provided Game Script and save game.
Crashing save: test_9720.zip
AI being used: Test AI #9720.zip

When run with -D ::1 -g path\to\the\above\save.sav, OpenTTD version 6953df7 crashes during load with:
Assertion failed at line 94 of D:\a\OpenTTD\OpenTTD\src\ai\ai_config.cpp: strcmp("start_date", name) == 0

Crash log: crash.zip