Anybody know why this happens?
As the message says, the AFCAD in use right now, is not the one which this .INI file was made for. This is very common if you have an airport with multiple AFCADs that can be switched, but you downloaded a customization made for only one of them.
You can TRY making a copy of that .INI file with a different name, which MUST follow the ICAO-xxxxx.INI ( xxxx are any characters you want ) naming convention, and JUST change the name of the AFCAD in the afcad_path line.
This might work, or not, depending if the parking configuration is compatible. If the different AFCAD only changed things like active runways, it will likely work, but if the *parking* configuration is different (parking missing or parking added), you will have issues, so a proper .INI file made specifically for the other AFCAD is required.
And, as explained in the GSX manual, Page 36, the full pathname is not used by GSX, only the AFCAD *name* matters
If you provide the scenery with different sets of AFCAD files, just provide several .INI files, named YourICAO.INI, YourICAO1.INI, YourICAO2.ini, each one based on a different AFCAD. The AFCAD pathname contained in the .INI file is not mandatory: if no file with that name is found in the full path, any other AFCAD in FSX matching the name only will be used instead, regardless of where it’s actually installed.