[asheshouse]

You must not have any new slot aircraft mods enabled which are not in your air.ini.
You must not have any aircraft listed in your air.ini which are not enabled.

I presume you have Mig15 v3.1 (new slot)

This line of text needs to be added to the basic air.ini

MiG-15 air.MIG_15 1 r01 SUMMER

Did you add the text to air.ini, plane_ru.properties and weapon_ru.properties for Spitfire Mk1?

Have you had any other new slot aircraft installed?
Are there entries for these in air.ini, plane_ru.properties and weapon_ru.properties?

Either disable mod aircraft and remove text entries
or enable and ensure all text entries are correct

Its best to only add one mod at a time so as to keep track of changes.
Keep backups of all files being changed.