Ender 5 S1 with Sonic Pad: blobs in circle prints

Ender Series Creality Ender-5
1

Hi

I just upgraded my Ender 5 S1 with a sonic pad. So far the print quality rose significantly!
Apart from 1 major problem:
Whenever the printer head is moving in circles, it stumbles or nearly stops and produces blobs.
This happens nearly every layer in random positions.
All straight lines or bends and arcs that are not exaclty circular are perfect and blob free.

Any one an idea?

2

What sort of retraction distance and speed, thinking it might be too low. Which slicer?

3

If you have made any upgrades of the stock items then changes are needed to the config files.

4

Using Cura 5.9.0
tried 40 and 30mm/s with 0.8mm as I found both recommended for Ender 5.
Retraction is fine everywhere BUT NOT in circles.
In some layers they are good, in some the printer head stumbles and generates the random blobs, sometimes 1 per circle sometimes more.

5

I use the configuration for the Ender 5 S1 that comes with the Sonic Pad, so already changed compared to the stock ones.
They work completely fine, BUT NOT for circles … Where’s the difference then?