IntelliJ IDEA 9.0 Milestone 1 build was released as an early preview for the new features coming in the 9.0 release (scheduled for November, 2009).

This build is intentionally limited to run until 1, October and then it expires to prevent our users from running an old preview version of the software.

At the moment you have the following options:

1. Switch to the 8.1.3 release.
2. Switch to the 8.1.4 Release Candidate (which will replace 8.1.3 soon).
3. Update your 9.0M1 build to the latest 9.0 EAP build.

Current 9.0 EAP builds are limited to run for 30 days since the date of the build, even if you run with a valid 8.0 license. New builds are usually published every week.

When IntelliJ IDEA 9.0 is officially released, you'll be able to buy an upgrade license  (if you have a license for any previous IntelliJ IDEA version) or the new 9.0 license to continue using the 9.0 IntelliJ IDEA version.

IntelliJ IDEA licenses purchased/upgraded after October 1st 2009 will work with IntelliJ IDEA 9.0 release.