looks like something is fundamentally wrong

are the leaves powdery grey, not sure in the pic.

could be heavy case of mildew

I think I would lift it and check for things eating on the roots,

in the worst case it is some bacterias/ fungus blocking the vascular tissue.

what worries me ist that the whole plant goes limp even though the soil seems moist,

fingers crossed, bye, Lin

Hey Lin,

Thank you very much for your response.

As you suggested, I lifted the dahlia this morning. When I did this, I noticed the stem was moving way too much. The stem itself was injured, as you can see in the picture. So I was forced to remove it. The tubers looked very healthy. Luckily, this one is forming some new growth. So there still is hope for this guy.

Thank you very much for your help!

Thomas