Salesforce OAUTH Error "We can't authorize you because of an OAuth error. For more information, contact your Salesforce administrator"

 Salesforce oauth_error_code=1800 is generic error which say OAuth flow has been broken.

The below list contains a few reasons when we see this error message

1. Sometimes corrupted cookies can cause this issue. Test the OAUTH flow by clearing cookies or using Private window of the browser.

2. Time out issue : Oauth login has 15-30 minutes of grace period during which Oauth approval process has to be completed. Otherwise this error can happen. 

3. Lack of necessary permissions such as API, Apex, Visualforce permissions for the user

4. If OAuth and SAML is involved with the flow, The error usually indicates that there is relayState issue.

5. Any other internal server error at Salesforce platform which can be solved by raising the case with Support.