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[Mollari]

MusicBee v2.5.5721.12459 (Win10.0), 18 Dez 2015 23:28:

System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei #=qj6Wupcc54CHoWGi2OLCMxZPcC4UgLnLcUamj5rqCnM8=.#=q_TUdVnUU1$xrEhlvDdJg9w==(#=qnKSyPDk904S4O8hdQm8SQ8pUC2ggZjx50tUSWMFqw0zADhrZ29G4N3CiYfU0DWPl #=qs0xg0vl5NaBQoKzBiob4kA==)
   bei #=q1brtWBldY4y9sdivHi59_qmq3TfelIoz30rTDy5Ucd8VAb1lS2scJgvOpjI3P3Fm.#=qRLBcTnxEa56BtbkBxr$ARg==(Boolean #=q5vegLOlsGo0KAu6SHEzHlw==)
   bei #=q1brtWBldY4y9sdivHi59_qmq3TfelIoz30rTDy5Ucd8VAb1lS2scJgvOpjI3P3Fm.#=q02Uzm95Xwq9C$Be97iCNTbNWZTNWLle00kc9T1lKi4k=(Object #=qCls3hrJAyptd6CD_6hB$rA==, #=qRx2i6EN7f0FJUuU$9e6YSOuEfca3ceg_1bvksUkSHWOg7pmLuK4n65y9vYq9zO5yp7Lkq8Ew2Msltc6KPatFLQ== #=qP2v$93GVCle37u7MvJMFxQ==)
   bei #=qsTyKatyJ_6Fur8aqUVX6Cv_XiECkYgNMqvCwHU5DghoPPToHyq3P0YsJXcjHXK$2.OnLeave(EventArgs #=qMun_6VF6Azngcl6iRoVmlA==)
   bei System.Windows.Forms.Control.NotifyLeave()
   bei System.Windows.Forms.ContainerControl.UpdateFocusedControl()

[phred]

That's a pretty old version you're using.  Try updating to the latest patch.  See my sig.
If that doesn't work, please let us know what exactly you were doing when the error occurred.  Step by step would be good so we could try to reproduce the problem.