Unable to open PDF Portfolios on iOS devices

I've created a number of PDF Portfolios for viewing via Apple iOS devices, which have all worked fine up until recently. Now, when I attempt to open any of these Portfolios - on iOS devices - the Adobe Acrobat Reader app just brings up a blank screen (with the name of the file at top); the only option given is "Done". I can no longer see or access any of the PDF files that comprise the Portfolio. I have reinstalled the app, but the problem persists. It all works fine on the Mac desktop apps.

As the ability to easily use this particular functionality on mobile devices is a core use requirement for me, unless I can get this sorted I'll just have to cancel my subscription to what was otherwise an excellent product. Any help would be very much appreciated.