It could be mechanical.
What you feel when rotating the axis can be a loose part in a bearing or a gear, and it can get out of the way for some time, but it will never go away.
Once in a while it will cause trouble and increasing damage.
Be sure to fix that first. It is a job for a specialist, I won't try it myself (and I try a lot!)
A company that helped us a few times is IRSA in Brunssum. (I am from the Netherlands too)
It can be the motor as well.
When is is (partly) shorted, it will cause this error as well.
Measure the resistance between the motor-leads and ground, they all have to be the same.
And it can be the servo.
One of the SIF's can be bad, or the powertransistor is busted.
Attached are two pages from my book. (in Dutch)