After upgrading Portal for ArcGIS, attempts to log into the Portal for ArcGIS website using a non-administrator account fail, and return the following error message:
Error: Error validating user: <Username>
The Portal for ArcGIS version upgrade is incomplete as the post-upgrade steps were not performed. The post-upgrade steps are done to authorize the users and apps in Portal for ArcGIS.
Log in to the Portal Administrator Directory with an administrator account, and perform the post-upgrade steps before logging in to the Portal for ArcGIS website using a non-administrator account. Refer to Steps 8 to 22 of ArcGIS Enterprise: Perform an upgrade of your portal deployment for more information regarding the post-upgrade steps required after upgrading Portal for ArcGIS.