Originally Posted by itimpi
In another thread Kovid indicated that the error message means that you have an invalid tweak somewhere. However as I also got it on a new installation I am not convinced as at that point I should not have any tweaks set! I have just been ignoring it and it does not seem to cause a problem.
Maybe simply opening and Saving the Preference: Tweaks: will fix the file.
(You might need to change a value for a moment to get it to trip the 'changed' flag)