Problems is still there (with Firefox, Chroome, Opera and Edge after clearing the cache), but only with coverarts and http. If I change manually to https, problem is gone.
Internet is rock stable (VDSL with 100 MBit/s down / 40 MBit/s up). Checked with different other image sites but they are ok.
Tried disabling Avira-Antivirus but nothing.