Additional information:

• the delete button in the 2nd screenshot is apparently disabled at the moment. The two exports buttons are responsive. The sync button appears to do the same as the automatic sync.
• I forced stop on the phone app from the android settings. Upon restart it resumes trying to sync.
• I examined the contents of the com.strava/files folder on the phone, and found 1 .fit file for today, I thought there might be another. I noticed that the file's last modified time (as seen on a Windows PC) says just a few minutes earlier, when the activity must now be more than 30 minutes old, so apparently something is still trying to write to it. The system time displayed on the phone is correct.
• I forced stopped the phone app again, deleted the .fit file, and started again. Still it attempts to sync forever, with the activity data still cached elsewhere. Possibly I will try to wipe the app data and log in again.

12 hours later - the phone app was still in this state. From Android's application settings --> Storage, the Clear cache button had no effect. The Clear data button worked.

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