I've had the same problem, you can usually get around it by pressing the escape key, it doesn't fix anything though it just means you don't freeze the screen. There are a few other problems as well, have you tried to enter a formula, sometimes you can't just click the cells you want to include into the formula, you need to type in the cell reference by hand.
If I quit and start up again I usually can save a few times before it doesn't work.
Tried reinstalling everything but still no go.
Cheers
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leon@matilda.net.au