Problem solved. We isolated the problem to a single line of code that exchanges the Authorization code for the Access Token. This is a function included in GitHub's Fitbit.Net. Updating the app to .NET Framework version 4.6.1 resolved this problem. However, there now seem to be some other (so far, minor) issues with the new Framework version. For example, a mouse HitTest no longer triggers an exception if there is a right click in a listview that has no items.
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Our desktop app is currently in development. The FitBit data download module has been working fine for about a year. Until a few days ago we could connect to the FitBit server, login using an email address and password and download weight and steps information. Suddenly on or about May 21, 2020, this module was no longer able to connect and download information from the FitBit server. The same problem occurs on different windows desktops and laptops using different browsers and operating system versions. At dev.fitbit.com on the OAuth 2.0 tutorial page, on the "Manage My Apps" tab, we tried clicking the generated authorization URL and got the same error; we never received the URL with the token. The C# code that performs the FitBit downloads has not been modified since June 2019. So what changed? Do the Client Id or Client passwords have expiration dates and need to be updated? Has the call back to localhost been blocked? Meanwhile we have a number of beta testers who can't update their FitBit information.
