Ils problem on B737 800

Did you set the frequency and the course? And in the final turn the vor/loc

I tried the same Barcelona 25R with the -800 on manual flight and the ILS kept switching from STBY to ACT even though every flight assistant option was turned off. It might be imposing whatever (unwanted) default route is present in the new Navigation Route menu page, say arrival to Runway 02.
The work around is to have 109.50 on both ACT and STBY so 27R ILS is guaranteed.

(I have a totally new Aerofly installation, maybe it is not set up properly).