I use your plugin on one of my blogs since month and never had any problems with it. Now I started another blog (same server) and always get the error that my keys are invalid. Checked them at Amazon – they are active and correct.
I now tried to simply save the settings on the old blog again without a change and I got the same error and everything disappeart and stopped working.
Err, I have no idea. Are the Apache configuration settings for the two hosts identical? You don’t get any other error messages, like your cache folder is not writable, and permissions are OK? Can you please check the XML files that get written to your cache folder and look up the exact error message from Amazon? Is the plug-in up-to-date? You know that Amazon changed their API policy and requires the secret key since July?
Could you make a screen-grab (I recommend the Firefox add-on Screengrab) of the entire administration screen?