Thanks Inculpatus,
I tried the flags -repair -verify -log to no avail. Then I noticed that in my install folder, every file had a "last modified" date of today, except for the d3d9_arcdps_buildtemplates.dll and d3d9.dll files which dated from 2019.
So I tried the -dx11 flag to bypass dx9 and that got rid of the nagging window ! Also, deleting the 2 dll above also worked without needing the -dx11.
TLDR : Got a bug? Delete old files not refreshed by an update, problem solved (YMMV).