I try to add a user on my On Premise System, but my credentials do not work when I try t open /p/register.
In our administrator manual we advise to limit the access to this path to Tomcat Users with a certain role. When you are asked for credentials opening the /p/register path, your administrator did follow this advise. Please ask him for the credentials. In case you had an administrator change and the credentials are unknown, please see for details on how the role was created: Configuring HTTP basic authentication