Interesting. I am not specifically using SCCM or your Software Center as shown in your picture. Does SCCM invoke the standard Windows Update under the covers, or is it doing something else to get the updates installed? My issue is happening when running Windows Update on the client PC.
My environment is using SCUP (6.0.394.0) to import the updates from the catalog, and then WSUS Package Publisher (1.3.1702.10) to approve them. I believe this "used to" be the recommended way to use the PatchMyPC catalog - or possibly it was how I initially did the Adobe catalogs and then added yours into it, it continued working up until now, and I have been too busy/afraid to try your newer more automated method.
Edit: I was pretty sure this was a Windows 2004 issue, but *just this morning* I have discovered a Windows 1909 machine that is having the same issue with Dropbox 95.4.441 so I am thinking it must be something else but it doesn't occur on all updates. Other updates I have found are Adobe Flash Player 32-bit/64-bit Plugin 32.0.0.363 and Adobe Acrobat Reader DC Update 20.006.20042 (Adobe updates from their catalogs not yours).
Also seen failing on 2004: Apple Mobile Device Support 13.5.0.26 (x64); Apple iTunes 12.10.6.2 (x64); Apple Application Support 8.4 (both)
I just installed an old iTunes x64 on a machine running 1909, it updated to 12.10.6.2 successfully, that same update fails on my machine running 2004.
I am thinking that instead of "All Updates fail on All 2004 OS" this may actually be "Some Updates fail on 2004 but work on earlier OS"
I will keep my eye out and see if others are found, it may be something in the update properties. The next one of yours that I can get to consistently fail on 2004 but succeed elsewhere*, I will let you know. *iTunes 12.10.6.2