Whenever I create or modify an entry, or try to just rebuild the daily archives, I get this error:

Building date-based archive 'Daily20070422000000' failed: Build error in template 'Category Archive': Error in <MTCategoryPrevious> tag: You used an <MTCategoryPrevious> tag outside of the proper context.

I haven't changed or modified this template -- it's the default from MT. I get this error even after I refresh the template.

Any ideas? Thanks.

The Building date-based archive 'Daily20070422000000' portion of the error indicates that this error is occuring when Movable Type is trying to rebuilding the Daily archive type. This would suggest that you have associated the Category Archive template with the Daily archive type. As the tag noted (and others) are required to be in a category context, which the daily archives don't provide, you will see errors.

To check which template or templates you've associated with each archive type, go to the Settings > Publishing tab, under Archive Mapping. If you've inadvertently associated the wrong template with an archive type, you will need to remove that association.

If you intentionally added the Category Archive template to the Daily archive type, what was your aim? You will still need to remove the assocation (or ensure that the Category Archive template contains only tags which work in a date-based context), but there may be another way to achieve you goal.