Upgraded to LR Classic V10 and Adjustment brush now hangs

Running Catalina 10.15.6 LrC 9.4 worked perfectly with adjustment brushes and everything. Now upgraded to LrC V10 and the Adjustment Briush no longer works (hangs with the rendering indicator permanently on). If I click back one step in the LrC history it stops hanging and I can do anything else (eg Radial Filter or Graduated Filter. If I switch to another image in Develop it also stops hanging. If I go back to the image with the Adjustment brush open it hangs again!

This also happens whether I am using the Graphics processor or not. 

Anybody any ideas?

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