Integration Errors & Fixes

1.error=redirect_uri_mismatch&error_description=redirect_uri%20must%20match%20configuration
Updated connected app callback URL with the callback URL from Auth provider.

2.We can't authorize you because of an OAuth error. For more information, contact your Salesforce administrator. 1800 : There was a problem in setting up your remote access
Log off all the open source and target orgs and try to authenticate again.

Scenario :
If any changes in Auth Provider,When you try to save named credentials with latest changes while authenticating you may get this error if the target org is already open.

3.The authentication provider didn't provide a refresh token. If the access token expires, your org won't be able to access this named credential.
Go To --> Authentication Provider --> Default Scopes - Provide value [refresh_token full]in this and save and save Named Credentials

4.System.HttpResponse[Status=Unauthorized, StatusCode=401]
Authentication Status in Named credentials should be Authenticated for the Target org user [Integration user]
Verify the credentials.