Since you cannot publish via the standard "Add a tab" or "Share with colleagues" methods, try these steps to bypass or resolve the error:
1) Try the Web Client: Access Teams via teams.microsoft.com in an incognito/private browser window. If it works there, the issue is likely a corrupted local cache in your desktop app.
2) Clear Teams Cache: Close Teams completely, navigate to %appdata%\Microsoft\Teams in File Explorer, and delete the contents of that folder before restarting.
3) Manual Upload (Admin): If you have a custom app package (.zip), ask a Teams Admin to upload it directly via the Microsoft Teams Admin Center under Teams apps > Manage apps. This often bypasses client-side publishing triggers.
4) Verify Permissions: Ensure your account has the necessary permissions. A 500 error can sometimes mask a lack of "Project Administrator" or "Publishing" rights in your tenant or backend environment (like Azure DevOps or Power Platform).
If the error persists for more than a few hours across all platforms (Web and Desktop), it is likely a backend regression, and you may need to Open a Microsoft Support Ticket with your Tenant ID and timestamps of the failures.
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