Summary :

Unable to login to the browser with an error : 

We couldn't log you in. You may run into this issue when you use specific browsers or add-ons that hide the referer header for id.atlassian.com and other Atlassian sites. Check for these and then try logging in again.

Environment :

Jira Cloud

Diagnosis :

1) Check if the browser is outdated. 

2) Try accessing the instance from a different browser.

3)Try loading the page from an external network (non office network). 

4) Check if you have logged into browser with any account. 

5) Try accessing the instance from a different workstation.

Solution :

1) If the browser is outdated, upgrade browser based on  Supported browsers.

2) If login works from other browser, then try uninstalling and re-installing the problematic browser.

3) If login works from the external network , then Verify if Atlassian IP's needs to be whitelisted on the problematic network.

4) If you already have logged into browser with any account, then try to logout and login. Single browser can handle one Atlassian account until you are in incognito (chrome) / private (firefox) window.

5)Additionally, you could try clearing the browser cache , cookies and disable any add-ons / plugins to see if that is preventing the login. If the issues persists still, then engage support for further analysis by sharing 

HAR files for analysis.