The card in question has an interval of 1 day, which shouldn't happen during normal review. You say you're not using add-ons, and I presume you did not manually reschedule the card in the browser? My best guess in that case is that the interval has been changed incorrectly when you studied ahead using the custom study feature. Please try the new scheduler and then see if a card answered after switching to the new scheduler exhibits the problem (bearing in mind that the issue may still exist for cards you reviewed previously):
I rescheduled around 80 cards in order to decrease my review load for the
day and didn't know that it resets my intervals...
Now, I've imported a .colpkg from a few days before and everything seems
fine, I'll just need to do some more reviews the next days.
I am very grateful for your quick and uncomplicated help - Thanks a lot !
Anki is great, and I'm happy that there is such an active community.