Just a thought.

Make sure you have enough space on the drive that contains your TEMP folder (probably your C: drive)

Even if you have downloaded the files to a separate drive and are installing to a separate drive it's possible those files are being extracted in your temp folder before being copied to their destination, if there is not enough space it's possible that's the reason why the files couldn't be copied.

Sorry, lost track of this topic due to personal forum navigation issues.

Update:
It was definitely corrupted files because re downloading parts around the percentage area of failure lead to a solution.

So checksums would have probably helped ind finding the exact files.

Oh, also I could not get Galaxy client to run, somehow it always says wrong password. I use a password manager and pasted the same password in the browser and there it works. So I assume that's related to some other problem probably firewall related or wrong http codes or any combination of those. My only Win10 install is heavily locked off and I only boot it for a few games. It's also behind an additional NAT because it's a VM. A GPU is passed through. benchmarks scores are within standard deviation of bare metal.
Given the hardware requirements are properly met this is now surprisingly easy. That was the Reason I installed Ubuntu 20.04. It came with a lot of Kernel updates that make PCIe passthrough A LOT easier.

If I have forgotten to wrap up something please remind me. Again, sorry.

The browser plugin also seems to work now. Maybe it wanted a browser restart or something. I think I did not even restart firefox after the installation of Tampermonkey.

On Tampermonkey security:
Yes, I also have those concerns.
But they are mostly dispelled now. The plugin asked me for every additional address the script wanted to contact. Thumbs up.

The screenshot is just the thing I was talking about, so yes. Just redundancy in case someone asks for it. And this is exactly the reason I usually set everything to English which I apparently messed up this time

"Apparently some files did not download properly and now the installer tells me that it can't read some cryptically named temporary file. "

Re downloading just the broken files was the intent from the start. But that was the point for that I need to know which file was broken. Thus the request for checksums. Lacking those I went for the low tech manual approach of re downloading roughly where the installer failed estimated by the progress bar.
Maybe that's an implied suggestion already.
Put some text in the progress bar like:
copying from part x out of y
Since this approach has worked 2 times so far the bar seems to mostly 1:1 represent the copy process by part, or I got lucky, very possible especially if bigger blocks are used in the compression.

On my PC, I just right-click a file and choose Properties, and then select the Checksum tab.

That file is then scanned to produce a checksum value, which it displays ... MD5 etc.

There is also an input field, into which you can paste the MD5 you have gotten elsewhere (i.e. my list) for that file. It is automatically checked with the scanned result to see if a match, and reports the result.

Obviously you would need to do that for every installer file to check for any mismatches (corrupted files).

It would obviously take a while for all the files for this game, but worth it to know just which ones you need to re-download.

Correct, but that gets the "is" value but I asked for the "should" value.

Also the issues you mentioned. Obtaining checksum from files I have was never the issue, I need the "should" values for comparison.

Edit: Just noticed that you where the one who posted the values from GoG.

Thanks for that; i up voted the posts.

But now I got gogrepo running. So there is no need to reinvent that wheel by writing another script to validate the files against a list. At least for now. I have no clue how officially tolerated that is.