Depending on your Joomla configuration, when linking contacts in Trombinoscope Contacts free or pro to a contact view, those links may result in a 404 error or even open an un-rendered page. Usually, this happens when the contacts have no menu item to 'fall' into.
An easy way to diagnose the issue is to put your mouse pointer over the contact link in Trombinoscope Contacts and to look at the link that shows at the bottom left corner of your browser window. An other way to find out what the link looks like is to right-click on the contact link in Trombinoscope Contacts to show the browser's contextual menu and select Inspect. The browser will open its developer console and point to the element you are inspecting.
What you end up finding may be a link that looks like the following:
This may look like incorrect, but the link is actually well built. It tells the system that we want to open the contact view of the Contacts component for John Doe in category 4 (which is the id of the category Uncategorised, for instance)
The link could also look like this one:
Now, since this URL seems correct, why is it ending in an error rather than opening John Doe's page?
It mostly depends on which router configuration you are using for the Contacts component. A router is a little engine that creates and interprets the URLs needed to open pages of the component it is associated to. And in some configurations (like the new Joomla Modern router without ids), this engine needs to know a little bit more information: the menu item id.
To fix this issue, just create a menu item of type Contacts -> List all contact categories. We will call it All Contacts. Select the top level category the contacts are members of and, if necessary, hide it by setting, in the Link Type tab, Display in menu to No.
Now, the URL changes to a more SEO-friendly form and does open properly: