After updating to 6.5.1.566335 (64bit) I noticed some of my inventory folders were empty. I cleared cache from the menu and re-logged and got the below error message like OP.
Tried other viewers and had no issues logging in and no inventory problems.
Tried the official viewer again on separate days, same issue.
Tried manual removing the ..\AppData\Local\SecondLife folder and the issue persisted.
Like the OP I uninstalled the Official viewer and manual deleted/archived all AppData folders and Application folder and did a clean install. Viewer works fine now.
I restored my ..\AppData\Roaming\SecondLife folder to get all my settings and chat logs back and the viewer still works. So I assume maybe a corrupt upgrade?
Of note though this release does include this in the notes.