FM15 does not work with files hosted from version 12. The minimum version is 13.0.9. This is due to security changes that have come out since version 12.
You either need to also update the computer hosting your file, or downgrade back to an earlier version. I would highly recommend the former due to the security updates.
FileMaker Pro 15 and FileMaker Pro 15 Advanced can connect to the following hosts:
FileMaker Server 15, 14, 13.0.9
FileMaker Pro 15, 14, 13.0.9,
FileMaker Pro 15, 14, 13.0.9 Advanced
The latest updates must be applied to the above software before hosting FileMaker Pro 15 apps.