An infinite login loop might occur if the count of logged in users exceeds the count of the maximum allowed users defined in the license. For example, if a user has a license for two content authors and tries to log in to a Sitecore XP instance in three different browsers at the same time, the logging-in with the third browser results in a redirect loop instead of displaying the kickoff user screen. The following message might appear in the log:
WARN Protected page accessed with no current user
To resolve the issue download and install the appropriate hotfix available on this page:
Be aware that the hotfixes were built for specific Sitecore XP versions, and must not be installed on other Sitecore XP versions or in combination with other hotfixes. In case any other hotfixes have already been installed on certain Sitecore XP instances, send a request for a compatibility check to Sitecore Support.
Note that ZIP file contents need to be extracted to locate installation instructions and related files inside it. The hotfixes must be installed on a CM instance and then synced with other instances using regular development practices.