El Capitan Finder shows 10 GB more disk space
I tried out the usual Apple voodoo things (PRAM reset etc), and one behaved strange: when I started the Macbook with pressed Shift-key (safe start), it took very, very long. Usually this takes like 5 minutes, this time it took more than 40 Minutes (huh!).

After that I just restarted again and everything was fine. So it looks like safe start fixed the problem