Hello,
The Drum unit indicator is flashing (only this indicator is flasing others are dark) . I've changed the drum unit but that didn't solve the problem. What else can be done, to be checked?

Thank you in advance!

Suggestion:

Go back to the Drum R/R instructions or the User's Guide . Replacing the drum may involve the manually resetting the drum's counter.

I don't have this model of printer but that's usually the problem.