Here you go:
Make sure you have a backup of the catalog before trying this.
Make sure Lightroom is not currently running
Make sure you only empty the folders, don't delete the folder itself.
1. Sign out of Adobe Creative Cloud by going to the preferences and sign out.
2. Exit the Adobe Creative Cloud app.
3. Goto finder and to the following folder //Macintosh HD/Library/Application Support/Adobe/SL Store folder.
on windows something like c:\Program Files\Common Files\Adobe\SLCache\ (or maybe (x86)
4. Delete the files inside this folder.
5. then look for the //Macintosh HD/Library/Application Support/Adobe/SL Cache folder.
6. Delete all the files inside this folder.
7. Empty your trash
8. Launch Lightroom
9. You will be asked to sign in again. click Not me then enter your adobe id and password.
10. You can then run and sign in with the Adobe Creative Cloud app.
(Steps are edited from a post by Graeme Bunyan on the Adobe Community Forum but are identical to steps I've followed under guidance from Adobe tech support)