Thank you very much.
It really seems to be related to liquid surface water (at least it seems not visible while the water is frozen). Might be something in the water texture.
I uninstalled the application and did a clean install, but the visual glitch remains.
It strongly reflects the ambient light so if you decrease that, it becomes rather faint but still visible.
Edit: I need to correct myself. It does show with frozen water.
Also btw, planets still start with a temperature at absolute zero, but since the effective temperature is correct, that's not a big issue as long as Albedo doesn't change automatically.
Did I mention that I love you for making this simulator a real thing? If not, feel hugged, if I did, feel hugged anyway