I had a closer look at my 1937 Carl Zeiss Jena Sonnar 50 1.5 (non T*). I found out, that there has been lots of tiny, tiny oil bubbles sitting on the glas element next to the aperture blades (on the rear tripplet). This oil was almost invisible to the naked eye. The lens looked very clean, the oil was only visible under certain lighting and in a certain angle.
So I cleaned the Sonnar rear tripplet and the aperture blades.
What can I say, the results are night and day:
flare is reduced by 70%, the contrast is better wide open, and the nasty color shift (slightly orange) is completely gone too!
Testshot on my A7II, f2, ISO200 (don't bother about the perspective):
I am blown away. Keep in mind that this is an 80 year old lens!
So for me, my search for an alternative is over - I will stay with my prewar Sonnar that I love.
Funny side fact: the lens had a certain soft look prior to cleaning that I really liked. This look is gone now. For portraits, it might be too sharp in some situations.