When a user adds a new item to a list, for example an Announcement, the name of the users is registered aswell. If these items are shown on a page using a webpart, like the default Announcements on a team site, the username is shown also. It is possible to click the name of the user to request more user details.
On an environment of a customer, a "Page cannot be displayed" error message is displayed when clicking this item. After some troubleshooting and reading through the IIS log, the cause of this issue is the following:
The link of the page that displays the user details is http://<servername>/sites/<sitename>/_layouts/1033/UserDisp.aspx?ID=<id>. This aspx file requests the aspx PortalAPI.aspx. The PortalAPI request the "Up To Link" URL and tries to contact the configured portal. In our case it turned out that users configured the Up To Link to a team site instead of the portal. Therefore the PortalAPI tries to generate a non existing URL, causing the Page cannot be displayed error.
So if you run into the same issue: First check if the Up To Link is really a portal!!