Do a hard refresh on your browser window:

Clear your cache or browser cookies.

Check the browser console when attempting to authenticate to check for an error that might indicate what is wrong.

Check your browser extensions to see if any of them are blocking authentication, in particular any ad blockers.

Try authenticating the connection again.

Make sure that your credentials are current, correct, and have not been removed.

Try testing your credentials in the system you're connecting to. For example, if you're having trouble authenticating the SFTP Connection, try connecting to the server with a different client. If the authentication also causes a problem there, the issue is related to the server, not BlueConic. If you can connect to the server with a different client, the problem may be related to BlueConic.
Note: Authentication errors with Facebook Advertising Connection can occur if you have not accepted Facebook's terms of service via your Facebook business account. Learn more here.

Reset your login credentials and try authenticating the connection again.

Log out of the external system and log back in.

Try authenticating the connection again.

Check your network security settings to see if a VPN or firewall is blocking the authentication.

Check the console window or task manager for errors.

Try authenticating the connection again.

Log out of BlueConic.

Log into the external system in another browser tab.

Then log back into BlueConic and try to authenticate again.

Make sure you enter your credentials precisely, with no spaces at the beginning or end of the string, correct URLs, etc.