As @JVincentBW does, I most often open CC via a bookmark, (contentcatalog.autodesk.com for me, but I'm not in the .eu), but I have used the app as well as launch in browser with no issues. Only Edge and Chrome are supported browsers.
Were you exporting from a project file or using the browse button or dragging and dropping to upload the files? Any method should work, mostly just curious.
Of the files that failed how many were updates to files already in CC? So what percentage of that ~50% were overwrites?
Unfortunately, I'm going to have to refer you to a support ticket again. Now, when they send it to the developers though, they'll be able to look in your back end of your hub and see if they can see any issues going on there. I'd recommend giving them your hub name and acc account number in the ticket from the get go. They will definitely need that info. Your ACC Account ID number is found under Account Admin>Settings.
Ric Weber
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My Ideas: https://forums.autodesk.com/t5/forums/recentpostspage/post-type/thread/interaction-style/idea/user-id/12292525