Getting the same error, tried in administrator mode as well as dragging and dropping the .map file I wanted to upload onto the .exe without success. Remember having the same problem with the Megaton Edition though.

yeah would be sweet to know how to upload maps because Im having the same issue.

Same just updated my map & can't upload it, I even put in the new weapon & monsters.

https://forums.duke4.net/topic/8853-duke-nukem-3d-20th-anniversary-world-tour/page__view__findpost__p__262462

MetHy solved it on duke4 forum. (and it works).

For the uploader to work you need to rename it to duke3D.exe and pretend to launch the game through Steam.

So in other words rename or move duke3D.exe, then locate DukeWorkshopUploader.exe and remame that to duke3D.exe, then from the library page right click and launch Duke3D.

I knew the solution before he even knew about it, but thank you anyways for writing here. It will be extremely helpful for everybody else who wants to upload maps until Gearbox fixes the tool.

If this is true, and I cast no doubt, kudos to you m8.

Good to hear there's a fix. It's insane that we'd have to do this though, Steam has had the ability to include multiple launch options for single library entries (accessible by right clicking) since at least 2009.