How to fix the "400: Session Invalid" error on Instagram?

The "400: Session Invalid" error typically occurs when there's an issue with a third-party app's connection to Instagram. Here’s how to fix it:

1. Open Instagram in a browser (mobile preferred as some users report better results).
2. Navigate to Settings > Security > Apps and Websites.
3. Look under both the Active and Expired tabs for the third-party platform (e.g. Postly).
4. Click Remove or Disconnect next to the app.
5. Return to the third-party platform and try reconnecting your Instagram account.

This process will reset the connection and typically resolves the issue.