Herbs: thyme dying back from bottom up, 1 by Peckhaus

Peckhaus wrote:
I planted a healthy-looking thyme plant in potting soil about 3 weeks ago. Within the last week, it started dying back from the bottom up and the dead parts look very dark (the darkness doesn't show up in the photo very well, but the rotting is clear). The plant gets about 6 hrs of direct sun per day and the soil drains very well. I've had the same problem with thyme (different varieties) in other locations around my house.

Can anyone help me figure out what is happening?

Thanks!