External FX causes Audio Performance Peak and audio crackle

Hi
When I insert an External FX I get very high performance Peaks witch result in audio clicks

Take a look at the Video (there are no sound in the video).
https://youtu.be/Ftnbsh5EsOo
From no Audio Performance Peaks to massive Peaks….
Its get less problematic audio wise if I increase the buffer, but I’m unable to use any lower buffer than 256 ,and the I have to accept clicks …
Any idea why this happens?

W10
Intel i7-6850K CPU @ 3.60GHz
32 Gb RAM

Cubase 12.052

All the latest updates
RME UFX+
FX send and return on analogue out/In 7/8

LatencyMon reports no problems (less than 250)