Hi NeTz3d, that is just noise, you have to keep rendering unless the noise is less visible, this may take ages because you have a lot of glass in there.
There si nothing to do, unless you want try to reduce the GI quality or try to clamp your GI.
Keep in mind that the denoiser is not a miracle tools, and is not keeping a regular behaviour through multiple frames, that is not flicker is a random pattern of the denoiser.
Alternatively if you use AE you can beneficial of using some tool for denoising that is based on multiple frame in order to keep the cleaning process consistent.
Ciao!