A user should have at least Viewer license to be able to login to Tableau Server. As per above mentioned workaround, if a user is unlicensed and assigned to a site, he/she won't be able to login to Tableau Server either using Desktop or Web Browser.
Viver, great and correct answer. Just for clarification sake, every user has to have a valid license on Server as either Creator, Explorer or Viewer. If they are not one of those categories, the user won't be bale to login. That is by design and for security reasons. When you have Tableau Server, you have to purchase a license for each user that will be coming, even if the user is just looking at dashboards (License = VIEWER). This applies to User Based License agreements. If using CORE based license, then the GUEST account is used for users who are browsing.