I just tried to send a FAX on openSUSE Leap 15.0 as a normal user:

$ rpm -qf /usr/bin/hp-sendfax 
hplip-3.17.9-lp150.3.2.x86_64
$ hp-sendfax -u

HP Linux Imaging and Printing System (ver. 3.17.9)
PC Sendfax Utility ver. 9.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

warning: Reportlab not installed. Fax coverpages disabled.
warning: Please install version 2.0+ of Reportlab for coverpage support.

I selected a .pdf file generated by LibreOffice using the "Add..." button,
which then resulted in the following:

Traceback (most recent call last):
  File "/usr/share/hplip/ui5/sendfaxdialog.py", line 375, in
FilesTable_fileListChanged
    self.toggleCoverPage(self.FilesTable.isMIMETypeInList(MIME_TYPE_COVERPAGE))
  File "/usr/share/hplip/ui5/sendfaxdialog.py", line 328, in toggleCoverPage
   
self.CoverPageGroupBox.toggled[bool].disconnect(self.CoverPageGroupBox_toggled)
TypeError: 'method' object is not connected
Abgebrochen

This works on my other (dual-bootable) openSUSE Leap 42.3 system.