Hi
my solution
install blender v2.91 experimental build as of 15_09_2020
installed not on c drive
did not take set up from v2.90
installed as admin
additionally
make sure hi res and low rez have render flag on if off fails
set bake to an actual folder path not c\tmp prefarably not on c drive
save as rgb 16 bit no alpha
set samples to 1 and 1
make sure bake extrusion big size here 2m, but if overlapping geo smaller size.
create new texture for normal map alpha off 32 bit on,will save as 16 bit later
assign mat to lowres, then image map load your new empty normal map button left
set to non colour
DO NOT link up image map node till baked
sel hirez geo then ctrl sel lowres geo, sel image map in shader editor as well
BAKE used def tile 32 32
not sure why unstable in v2.90 maybe c drive permissions,
or blender v2.90 taking settings from v2.83
https://www.flickr.com/photos/93465359@N03/50343650117