Just for the record
I just finished combing the modlist, it was an outdated slider mod. I updated it & the pop up is gone now.

Again, thanks for the help.

Do you have a link to the slider mod in question, if is still actually available anywhere? I'd like to grab this thing and see the error in my own game because if we're going around telling players that only these five or six Core Mods can trigger it, and that is not accurate, then we require more information to better guide others.

Right, my bad, I should've mentioned the mod name.
The mod name is "XCAS core mod: more tattoo locations, edit naked outfit, slider hack, body hair, more - update 5/23/16 Steam version added." You can find it on modthesims.
My archived version is 1.67, I updated it to 1.69 so everything is fine now.

Yes, I use that mod as well. You might want to take a look again at the list of five Core Mods that I said were in popular usage and which we know are capable of throwing the core version mismatch warning. I'll put it in bold here, sorry that my listing it out didn't really save you any time in narrowing things down.

igazor post_id=77007 time=1599867796 user_id=6716 wrote: NRaas ErrorTrap
NRaas UnstranslatedKey (there is no 1.69 version)
CmarNYC's XCAS
Consort's 2X Weight/Fitness
AwesomeMod (current version auto-detects 1.69 and reacts accordingly)

Oh I didn't know it's the same one. Stupid me.