Looks like the whole service is in maintenance (not just the API). I just visited the website (ChipMixer.com) and tried to create a new session. The same message appeared for me.
Just wait a few hours.

Ok

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New version, 0.0.3. Now randomly selects two of Bitmix, Privcoin, and Foxmixer. Both the mixer selection and ordering is random.
To upgrade:
pip3 install -U doublemixer