The app stops working

I have been using the app for 3 1/2 years. I only use It for walking. It has worked with no problems until I changed my phone from Huawei to Oppo about 3 weeks ago. Since then when I try to use the app the following happens.
The app gives me the 5 second count down and then starts normally. While I am walking, I get the 250m verbal progress announcement.
When I have completed the walk, the app has stopped working. I haven’t touched the phone for any reason. It stops working after about 400m.

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