Hello doclucas,
You are surely right about Anti-Theft version problem (cannot test here because all my devices are updated).
But at this time, Anti-Theft version did not change since 5 month ago (2016/10/04 for release 4.2.0.8278), but this problem from my.avast.com came only yesterday !
And we do not have any updates history for my.avast.com website …
So I think that we can affirm now that this problem comes from a compatiblity issue of the last my.avast.com update (?) and the last Anti-theft version (4.2.0.8278).
Thanks for your help, but as you said, now all we need is the right direct support channel to contact and address this !
Thierry
NB: of course, I didn’t get any reply yet after my last reply yesterday evening to Avast Support Team.

BUT : to confirm tolksee last post (while I was writing this one), I can confirm that all Android devices are back on my.avast.com at this time for me.