GTDInbox relies on a key piece of information in Gmail's internals for some of its more advanced functionality, such as creating/renaming labels (at your request). Occassionally, this piece of information will move and GTDInbox will lose some functionality.

GTDInbox can find the information it requires, but it's an intensive process. So we left it for you to decide when you run it. It may take several minutes, but it is completely harmless. To begin, please click 'Fix Gmail AJAX' below.

GTDInbox could not fully read the threads (your emails) from Gmail. This means that certain GTDInbox features, like auto-labelling and previews, will fail.
It is often caused by either changes in Gmail (unlikely) or a clash with another addon. Often, a restart of either Gmail or Firefox will solve it. As part of a long term plan, we're working toward a day where this problem will never occur again.

You can read more at either the FAQ or you could scour the blog.