I first tarcked this down to the Comments-field of - what? The SitePages library versioning. So I had two workatrounds to offer the customer: disable page versioning or always check out/in pages, which asks for the comment, thus actually asking for the required value.
This however was just a workaround, not satisfactory enough, so some real solution was called for. Googling, once again I managed to acquire a solution that was so simple in itself, yet so farfetched that I was real sceptic about it. But it worked!
In a nutshell, the reason for this error was the hiding of the PlaceHolderPageTitleInTitleArea. I had moved this place holder in my master page to the hidden panel, with opening tag:
asp:panel visible="false" runat="server"
This for some reason resulted in the error, but when changing the visibility setting to be done by a CSS rule, the error disappeared. Simply, add this CSS class, eg.
Then replace the visible="false" attribute in the asp:panel tag by cssclass="hiddenpanel" attribute. And that's it.