I finally found the official release note of Avast through Google
(the same link that DavidR provided in the meantime…, thanks anyway)
https://www.avast.com/fr-fr/release-history
So Avast 8.0.1497 was indeed meant to work on Win8.1 64 bits
(which does not mean that all compatibility issues were corrected in this version)
Still strange that the windows event log reports it as being incompatible.
Since no one seems to have tried this combination (Avast 8.0.1497 on Win8.1 64 bits)
I will have to test it myself :
At least I can say that this Avast intercepts (false postive ::)) virus in 32bits apps.
I already got the Evo-Gen false alarms when compiling some of my 32bits programs.
I will have a look on this thread within a few days, in case someone has already tried what I’m doing.