In the charts Salt Pond Bay is told as day Anchroage but what about overnight anchorage? Is it forbidden to stay there for the night?

E. g. today the spot was not rolly at all and we left the island only because we were not sure if it might be forbidden to stay there for the night.

Salt Pond Bay (north shore of Salt Island) is a fine overnight anchorage as long as you dont't have strong winds from north of East or heavy north swell forecsst to move in overnight. I can't think of anyplace in the BVI that you are forbidden to overnight anchor (except for within mooring fields and the National Park boundaries which are no-anchoring at all times).

Salt Pond Bay, St John is a wonderful and secure anchorage (mooring balls only) in virtually all conditions.

Thank you very much. I therefore wonder why navionics assign this place as a day anchorage as normally every sailor is responsible by themself to check the upcoming conditions...

It’s not well protected. You need the wind to stay out of the east and a squall at night would not be fun. Wind out of the east and no squalls forecast it’s fine.
G