Facebook Connect login no longer working in IE

Facebook Connect login (using their FBML javascript widget) has been working fine for us on Firefox, Safari, Chrome and IE on both windows and mac. But IE has stopped working :(. I get this error message now on the popup box that comes up when you click the facebook login button:

API Error Code: 100
API Error Description: Invalid parameter
Error Message: redirect_uri URL is not properly 开发者_JAVA技巧formatted

What happened and how do I fix this?

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Try updating your flash player as suggested on the Facebook Developers page here