I also sometimes get that, you just have to close the window, then open it again and try to login. (you have to try it several times until it works)
Have you ever been able to login to the Launcher?
Can you login at https://unrealengine.com?
If you’re able to login to the website but not the launcher, then something funky is going on. If so, can you send logs with logging set to verbose, as described here:
enable verbose logging by editing your DefaultEngine.ini, which you can find here: "c:\program files\unreal engine\launcher\engine\programs\noredist\unrealenginelauncher\config" and add the following section to the end: [Core.Log] LogHttp=log LogOnline=verbose After you make that change, can you run again, and then post the logs here? If possible, zip up the logs (c:\users\YOUR_USER\appdata\local\unrealenginelauncher\saved\logs) and post the zip here.
The next time anyone has this happen, please try it with the “verbose” logging that Eric requested and then upload the logs here. This is certainly an issue that we would like to work to resolve.