Thank you 
Reconnecting the earbuds had not resolved the problem with the left earbud before I went to sleep (the right one was working). Today, after having charged them over night in the case, the left one works fine again.
I did notice that; it says 100%, even though I think that the often-used earbud can’t be fully charged. Maybe it shows the battery level of the more charged earbud.
What experiences did you make regarding the charging and LED behaviour (especially when often only one earbud is used)?