Users got the error message "Your licence for Office Server Search has expired" when performing a search or when opening a page which contained a web part that used the search.
The other day I had to install some language pack on a MOSS2007 environment. After installing a language pack, you need to run the Configuration Wizard. But because people were working on the environment and the Configuration Wizard stops some services, I waited until a more suitable time to run the wizard.
While troubleshooting the issue I noticed that permissions on the registry were gone. So MOSS2007 wasn't able to read the license information and therefore reverts to expired mode, presenting the users with the error mentioned above.
Putting two and two together: Installing a language pack resets some permissions on the file system and/or registry. This is corrected when you run the Configuration Wizard.
After running the Configuration Wizard, all problems were solved.
Do not install language packs when you cannot run the Configuration Wizard immediately afterwards.