Rotating circle on content manager.

It's just this. Also, the green bar at the bottom is moving quite fast.

This happened after I attempted to install a NON-DLS dependency pack for one of my paywares.
Build version is 17026

This is normal as the database is updating. If you click on developer menu above, and then choose Show logs, you'll see status information on what's going on.

Thanks! Also...
I had to go, so i closed the game, came back and reopened it and a database rebuild opened.

Also, its been 2 hours, and checking the logs, the rebuild hasn't gotten pastan asset called ; <NULL> : TrainzLockNativeFileSystem> resumable lock timeout: c:/users/(myname)/appdata/local/n3v games/trs19/build bi5jyyh71/packages/1068.tzarc ()

Not sure what this could be.
Possibly your AV software is preventing access?
1068 is a previewpass item. Maybe try uninstalling and reinstalling that package.
If you can't uninstall using Content Store, delete that file from /packages folder and rebuild the db.