Update - We have identified and mitigated a problem which was causing the Fitbit API to return http 500 errors in response for requests for heart rate data and exercise details, typically for about a 5 minute period at the beginning of the hour.

The root cause problem is that an extremely small subset of users have certain large-cardinality data which was being queried repeatedly, expensively, during periods of high API traffic. Those database queries were overwhelming a shared database and causing problems accessing data for other users.

We have mitigated the issue for most users, but requests for a small fraction of data -- approximately 0.02% of all requests -- may still experience http 500 errors or timeouts. In cases where you see an http 500 error, trying again may help. We're continuing to work to improve database performance and will publish an update when we have made progress.
Aug 27, 10:18 PDT

Investigating - Users of our api.fitbit.com API endpoints may notice an elevated rate of http 500 responses to requests, beginning at approximately 2020-08-23 08:00 UTC. Requests for heart rate data or activity details are most likely to be affected; requests may return an http 500 response instead of the expected response up to 1% of the time, for short periods of about 5 minutes, typically at the beginning of an hour. We're investigating the problem and will give further updates when the issue has new information or has been resolved.

In general, retrying the affected API call is likely to succeed, and waiting a few minutes and retrying again is very likely to succeed.
Aug 25, 15:03 PDT