[REG 5.9.2 -> 5.10] Cannot print from any Qt-based application to printer or cups-pdf

After upgrade to Qt5.10beta1:

• I cannot open localhost:631 - I've get this message: "This site can't be reached The webpage at localhost:631 might be temporarily down or it may have moved permanently to a new web address. ERR_UNKNOWN_URL_SCHEME"
• It's not possible to print from any application that is build against Qt5 (I haven't any on Qt4). When I try and choose my local printer (Brother) the field "output file" is always "on" with some address to PDF file, and of course it's not possible to print to this pdf; after delating it, and this field is empty, I've got a message: "The file is a read-only file.
  Please select a different file name"
• When I've choose Virtual_PDF printer (it's related to cups-pdf), I cannot type any pdf file in those field
• I can print from any Gtk based applications (like GIMP) or Qt5 agnostic, like Libre Office
• After printing from i.e. Libre Office I can get access to localhost:631, but there aren't any logs from that moment, when I've tried print something from Qt-based applications.

More: the same bug reported on Arch's BBS: