I have created a Page Layout based on a custom Content type which inherits from Page Content Type. It contains four Custom Fields and a PublishingWebControls:RichHtmlField for each field. When I try to add web parts to it it only renders html as text. why You can add web part to a PublishingWebControls:RichHtmlField in BlankWebPartPage.aspx (OOB pageLayout) but it is impossible to add same web part to a custom pageLayout? thank you for your help.
Are you adding your RichHtmlField controls to asp:ContentPlaceHolder ID="Main" or you've created some additional placeholders? If the second is true for you that build your master page in such a way to place the Main placeholder as the last one of used placeholders.