In a main view use the key combo Ctrl+Shift+U and select "Reset Program Settings". Close and reopen RM8. Then go to Publish>FamilySearch Central and see if you can login. FamilySearch's FT API does not currently support two-factor verification. If you have that turned on with FamilySearch you will need to remove it. The FT API also supports limited special characters, some characters may cause a login issue.

If you are still unable to login temporarily turn off your firewall, antivirus and internet filtering software to see if one of them is blocking RM8 access to the internet. Then adjust their settings.