What does this error mean?

I suddenly noticed that i wasnt automatically buying GPs and that the world religions panel was blank. I checked lua.log and saw this :

[82626.834] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshContent'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:213: in function 'TabSelect'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:725: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:719>
[82627.770] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshContent'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:213: in function 'TabSelect'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216>
[82630.235] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshContent'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:213: in function 'TabSelect'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216>
[82635.835] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshContent'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:213: in function 'TabSelect'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216>
[82638.284] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshWorldReligions'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:714: in function 'WorldReligionSortOptionSelected'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628>
[82639.376] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshWorldReligions'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:714: in function 'WorldReligionSortOptionSelected'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628>
[82640.016] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshWorldReligions'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:714: in function 'WorldReligionSortOptionSelected'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628>
[82640.796] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshWorldReligions'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:714: in function 'WorldReligionSortOptionSelected'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:628>
[82689.749] IGE_PlayersPanel: IGE_SetMouseMode, mode=0
[82693.259] Runtime Error: Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: attempt to index local 'holyCity' (a nil value)
stack traceback:
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:450: in function 'RefreshContent'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:213: in function 'TabSelect'
Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216: in function <Assets\DLC\Expansion2\UI\InGame\Popups\ReligionOverview.lua:216>

Any idea what this is about?

is anything in the game being affected, like the holy cities?

Generally a 'Runtime Error' in the lua log means that the text of the referenced lua program does not have a format error, but while running in real time the program ran into an error condition. The fact that the errors all end with "attempt to index local 'holyCity' (a nil value)" leads me to believe that there is something clashing between activated mods and the Firaxis-Supplied ReligionOverview.lua. Most likely something caused the game to 'forget' the status of your Holy City being the Holy City, so when the game tries to look-up which of your cities is the Holy City it is getting a value of none (nil), and this sort of result is no-good for the ReligionOverview.lua. I've had this sort of thing happen to me before (the game 'forgets' which is my Holy City). If you are using Calypso's Colored religions mod you might try to not use it in a future game to see if this cures the issue. I am not positive Calypso's Colored Religions mod is causing the problem, but I can't think of any other popular mod that would possibly cause this.

Unfortunately I think you are toast for that game. I can't think of any expeditious method by which you can 'rescue' your saved game and get back your Holy City status, except perhaps going to a previous save and trying to un-enable Calypso's mod if you were using it.

Yea...i lost my holy city and i wasnt using calypso's mod. Hasnt happened since then, may have just been a one time thing...