I also tried it on Discord, exactly the same. My old Astro A50 runs perfectly. I suspect the problem is with the amplifier. It doesn't deliver enough power for the MMX 300's mic.

I am completely in despair, the sound is great only the mic is muffled.

I use the same MMX 300 with Sennheiser GSX 1000 and the mic sound in TS and Discord is very good, according to all my interlocutors who think I am in the same room. So it's not the hardware (unless it's defective?), it's much more likely that it's the mic settings in Discord and TS and something is not set well. All additional filters for the mic should be switched off, otherwise there will be chaos. You can also activate the feedback of your own voice in GSX to check how it sounds.

Hi!

I have a different headset, but I use the practically identical amp (GSX1200).

I could imagine that 2 microphone settings "don't like" each other somewhere. I know less about Teamspeak, but I know Discord. Do you also have various settings for your mic in Teamspeak? Maybe background noise attenuation? What about the Windows settings? Any mic boosts activated?
I would first try it out, sounds like a matter of settings...