Unable to choose a Conda environment as interpreter

PyCharm has suddenly started misbehaving with Conda environments.

None of the many ways of associating a Conda environment are working. For example, if I click on "<No interpreter>" in the bottom right, I am able to choose the Conda environment, no problem. But dismissing the dialog, the status is still "<No interpreter>".

Then, the IDE presents a message asking me to configure an interpreter for the project, which I do. But again, this has no effect and the message persists.