However, it can happen that when a user tries to look at these pages (often the Login page!), the following message or a translation of the same in the local language of the user appears:
"The page cannot be displayed because an internal server error has occurred".
You typically get this message when the inserted HTML is not XHTML compatible. There can be many mistakes in your (X)HTML, so it is not possible to immediately give you the reason why your customization fails. Therefore, please check HTML with online validator.
One common problem is the following (typically caused by creating HTML with an HTML authoring tool or starting with copying from another HTML file):
Open the HTML file with a text editor (Notepad, Notepad++, etc,.).
If it starts with the following line, remove the line: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Or comment it out: <!--<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">-->