This issue has been resolved in version 70.0.0 of the Strava app. If you update your app and continue to have problems, please let us know.
We are aware of an issue with the latest update and Watch OS 4.3. If you are able, please update your Apple Watch to version 5. WatchOS 5 requires iPhone 5s or later with iOS 12 or later, and one of the following Apple Watch models:
Apple Watch Series 1
Apple Watch Series 2
Apple Watch Series 3
Apple Watch Series 4
Unfortunately, watchOS 5 is not compatible with the first-generation Apple Watch. If you are using a first-generation (Apple Watch Series 0) device, you will not be able to prevent your Strava Apple Watch app from crashing. We are working on resolving this issue and appreciate your patience.
You can follow this known issue for email updates.