Musescore 2, and now 3, does not proceed to Import PDF after a file has been selected

On all occasions and using several different PC operating systems, ie Windows 10, vista, XP; both Firefox and MSN Edge/Explorer the same situation occurs.
My internet provider is Virgin media in UK and they say they do not block Musescore.

I login into Musescore 3 and use the facilities quite happily.
I have used the PDF converter happily for 30+ files in Musescore 2
Some 3 months ago the PDF files stopped being uploaded, without any error message.
I assumed the problem would be rectified in your usual quick fashion.
I have now updated to Musescore 3, lovely! no difference for the PDF problem tho.

I have tried to allow musescore Trackers and Cookies and McAfee says Musescore is a safe site.
After selecting the PDF file to be uploaded, the files window closes and returns to the previous window; the tab heading will flick and the McAfee shield will flick showing they have been activated, presumably. This will happen continuously (in a loop) until I interevene.
As there is no error presented and other Musescore facilities still function I am at a loss as to what more information I can provide?