I wanted to check out The Ruins Mod (https://mods.factorio.com/mod/AbandonedRuins) and started a new map. This happened:

The mod Resource Spawner Overhaul (6.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event rso-mod::on_init()
The mod The Ruins Mod (1.0.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event AbandonedRuins::on_chunk_generated (ID 12)
__AbandonedRuins__/control.lua:39: attempt to index field '?' (a nil value)
stack traceback:
    __AbandonedRuins__/control.lua:39: in function 'spawn_ruin'
    __AbandonedRuins__/control.lua:57: in function <__AbandonedRuins__/control.lua:43>
stack traceback:
    __rso-mod__/control.lua:1042: in function 'spawn_starting_resources'
    __rso-mod__/control.lua:1879: in function <__rso-mod__/control.lua:1875>
stack traceback:
    [C]: in function 'force_generate_chunk_requests'
    __rso-mod__/control.lua:1042: in function 'spawn_starting_resources'
    __rso-mod__/control.lua:1879: in function <__rso-mod__/control.lua:1875>

I'm not sure which mod should be fixed, but the message says RSO caused the error (even though it seems to come from AR's event handler) so I'm reporting it here first.

It seems that Abandoned Ruins mod is not prepared to be called while on_init in RSO is running.
RSO makes sure that area around starting point gets generated when spawning starting resources and it seems to break spawn_ruin event handler in Abandoned Ruins mod.
It needs to be reported to Abandoned Ruins mod.

Fixed in version 1.0.3 of The Ruins Mod (AbandonedRuins).