Received the beamer new yesterday and connected it today. Disney+ is available in the LG Content Store. However, when I press "Install", it says that it first needs a software update. When I confirm with "Yes", it says "Software update not possible" without further instructions.

I looked yesterday and yes -- the Disney+ app is not yet available in webOS on the projector itself.
However, I use the Apple TV 4K myself. Disney+ and pretty much everything else you could possibly need is available on it. With an HDMI 2.0 cable, you can get the full picture quality (4K with HDR10 + high-res audio) on the projector. So it would be a workaround.

Have you tried logging into Disney+ via the internal browser? It's certainly not the most sophisticated option, but it might work.

Hi, I can confirm that the Disney+ app is not available, nor is the Apple TV app. The projector arrives with WebOS 03.00.06 and no updates to this are available. The projector is advertised everywhere as WebOS 4.5, however, which is confusing and disappointing
https://www.lg.com/uk...

If you're feeling brave, there appears to be an upgrade on the US LG site to WebOS 4:
https://www.lg.com/us...

Since WebOS 3 is software for the 2016 models of LG TVs, I am not confident that it will get an upgrade, so will likely return, in spite of the picture being great. If you have another decent 4K source, like @Rstoiber and the Apple TV, then you are fine.