That was supposed to be fixed in 1.16. Unless you want to wait for 1.17, which I believe is being released on the 9th of this month, to see if it fixes it then unfortunately I don't think there is anything you can do apart from deleting your game folder and reinstalling. I've had this happen to me twice; once with 1.14 and 1.15. However, while this "fixes" the problem deleting your game folder will also delete any saved replays you haven't uploaded and also photos that you haven't exported to USB (or uploaded). PD, in their infinite wisdom, decided to store replays and photos with the game data, rather than with the save data. General settings, car setups and liveries that you haven't yet uploaded all survive.
If you do decide to delete your game folder note that, initially, after you've reinstalled GTS the thumbnail images of the cars in your garage may not appear straight away, but they will eventually be automatically generated again.
Which has just reminded me of a bug I don't think I've seen mentioned...
After reinstallating GTS, if you go to open an existing design in the livery editor, the thumbnails for the liveries are gone, they appear to be another victim of PD randomly storing things with the game data. Unlike the thumbnails for the cars in your garage these unfortunately do not automatically regenerate, but there is a, somewhat tedious, fix; To regenerate the thumbnail you have to open every livery and re-save it.
I don't think this will fix the problem. It is apparently a problem with GTS checking the data on the server (communication error?) and nothing to do with any files on the console. Rebuilding the database won't do anything.