I solved this issue (Raspberry Pi 2, Hifiberry DAC+). It seemed to be a problem with the cache. I limited the cache size to 20 MB and changed the cache path to the Raspberry Pi's RAM:
#SSH into your system
ssh -l root ip_adress
# open the spop config file
nano /etc/spop/spopd.conf
# limit the cache size to 20 MB
cache_size = 20
# set the cache path to the Raspberry Pi's RAM
cache_path = /var/run
#Save settings
After this, you have to restart the Raspberry Pi. Since then I experienced no clicks or pops while playing music from spotify.