If you contact Product Support, they can get you access to a beta version that adds a new panel in Build mode:
It’s available under the View menu as “Data Logging…”, and will only appear for Admin users. It’ll give you a list of the tags you’re scanning for trends, how much space they take up on your box, and buttons to download the raw data files (not csv or json, raw binaries) along with a button that will wipe out all data we’ve scanned for a given trend.
It’s really basic, you can’t sort things, there’s no feedback, and if something goes wrong it’ll just silently fail. (Though there might be something in the logs.)
If you can find the tags that are on trends stuck on “Loading…”, download those files and send them along to Product Support so I can take a look at them. If you choose to, you can wipe out the data for the tag and your trend should start working again. (You’ll probably need to refresh any clients looking at it though, or make a tiny change on the pages with dead trends to get the clients to refresh automatically.)