Nomore downloads and updates in iTunes

My iMac on OS X 10.11.4 (El Capitan) is up and running for years. Since several weeks I'm having two error messages showing up when iTunes is starting, saying there is a directory missing, but I dont know which one:

Error 1: Directory missing

Error 2: Trying to buy/download an App in iTunes App Store, this error is showing up:

Error 3: Trying to update apps downloaded in iTunes App Store, this error is showing up:

Settings in iTunes are ok, all music is there like configured

_/Volumes/Macintosh HD 2/Users/my user/Music/iTunes/iTunes Music

Any hints or suggestions how to solve this problem?

regards

beatus