Hello annika179,
Welcome to Apple Support Communities. It looks like iMessage keeps crashing on your iPhone. We understand how important it is to be able to communicate with family, and friends, and we'd like to help.
What do you mean when you say iMessage is crashing? Does it quit unexpectedly, stop responding, or show an error message? Please specify.
Are you experiencing this behavior with any apps other than iMessage?
What is the exact iOS 15 version installed on your iPhone? Settings > General > About.
We recently released iOS 15.0.1, so if you haven’t updated to that, let’s do so now.Â
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Also, check this out:
If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open
We hope this information is useful. Kind regards.