Dieses Problem habe es schon einmal früher und es wurden im englischen Teil des Apple Forums einige Lösungsansätze gefunden, zum Beispiel in dieser Diskussion der Lösungsweg von Lucerova
How to fix it:
1. Select Settings > General > Spotlight Search
2. Un-Check Messages in the Spotlight Search
3. Delete all text messages that show in the Messages app
4. Open the multi-tasking bar by double tapping on the Home button
5. Close ALL running applications
6. Restart your phone
*You may have to repeat this process multiple times depending on how much storage your Messages app is currently using. In Settings > General > Usage, You may only see the mb or gb drop by a few hundred mb each time (don't panic). You need to continue repeating this process until the Messages app is only utilizing 20mb or less. --If you get stuck and you still have a significant amount of data used by Messages and no Messages to delete, Simply backup and restore your device at this point to empty the remaining caches and allow you to permanently remove the remaining data.
**Also be sure to re-enable Messages in your Spotlight Search. Select Settings > General > Spotlight Search
Always ensure to clean out your Messages often, as well as restarting your device often. iOS has self maintaining functions that only run on startup. As long as you don't allow your message threads to over-populate after extended time periods, you will not need to repeat these steps.