This issue is not linked to itsme®. You can get this error message when you try to log in to government websites from a mobile web browser. Please make sure that you are using your smartphone's default web browser or that you aren't surfing in private mode. You can change the default browser in the settings of your smartphone. For Android we recommend not to use Samsung Internet but to use a different browser instead. This will avoid the error message.
Since the iOS 14 update, iPhone users can also change their default browser in their settings. If you still experience issues, please visit the website from a PC or tablet.