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I can confirm that this also occurs on the downloaded version.
Strange - because it´s just not working on North-to-South and East-to-West direction and vice-versa when the circle is 10x10 or larger. But when you convert circle to path first knife-tool is working again On smaller circles it´s working instantly.
Have a look here:
Thanks for reporting.
Would you please try it on an anonymous window on chrome (ctrl+shift+n) and check if it is solved?
It appears to work properly now, with or without anonymous mode active, on any size of circle (I was using a 300x300 circle when reporting this issue).