I'm terribly sorry for causing troubles Henri!
I found out while going through everything I could in the settings that I somehow had got the "extended environment" shaders disabled.
Re-enabling these in the viewer preferences solved the issue.
I guess too much of playing around with all kinds of cross comparisons with lots of different settings combined with tiredness before the holidays made me miss that detail...
Case closed. All good.
(And for anyone else, double check the shaders first of all!)