There was a problem when 6.0.1 was released that kept you from logging in a server. This was fixed by a hotfix that was posted in discord, but that hotfix has now been added to the download. If you downloaded this before the fix was included you may need to download again.
Also, if the server is building/creating a world it will not let you in until teh world has completely generated. Make sure that it shows your world has completed generation before trying to login or it will hang or disconnect you