Calibre jobs get stuck

I'm using Kindle PaperWhite and I recently updated Calibre to 1.15 x64. Now, everytime I open Calibre, there are 2 tasks in jobs tab I can't stop.



The main problem is that I can't add any file to my Kindle because that 2 tasks still in qeue.



Do you guys know how I can fix this?

Thanks for all.