Taskalfa 2551ci - White spots/smudge

Hello everyone.

We have a Kyocera TASKalfa 2551ci device. Usually it works fine, but sometimes when we print, a white smudge appears on the paper. This usually happens with blue color.
I attached a picture of the printout.

Its not always at the same place and not all the time.
I tried a Drum refresh and Calibration but nothing.

There is enough paint in the toner.

Any suggestions?

Regards,

Adam