I am having the same problem on both of my computers and it started when I tried to install the optional preview to this update on 8/2. However, on one of the computers I did not even attempt to install the preview updates because of the problems on this computer so I don't think that has anything to do with it. Sometimes it will do as you say and I get an error code indicating it is a .net 3.5 error and I have had it start to download and stop at 1% giving a download error code
All of my other updates download and install just fine without any errors including the .net preview on 8/2 and the .net update on Tuesday as well as KB4570334, the servicing stack update and the several daily Defender definition updates. I have run the reset windows update as mentioned in one response both as that batch file and manually, as well as the troubleshooters for windows update and BITS, and I have run sfc and chkdsk and none of these have indicated any problems.
I can't believe the problem is with Windows Update Service in itself, else why would everything else download and install as it should. And to enforce that thought I have, as you have, run the stand alone update and it will run like it is doing its thing and when finished say the update was not installed.
I am beginning to think there is an error in the update that effects only certain computers. This computer is self built and has been very dependable with hardware and software with virtually no issues. The Dell laptop is "from the factory" so I had nothing to do with the components in it. I am running Win 10 Home on both, and this computer is a four core Intel running at 4.2Ghz (4.5) with 16 gig of RAM on an Asus Maximus IX Code MB and the system is installed on an SSD plus it has two SSHD drives for data and backups as well as a detachable USB drive for reserve backups that are stored nightly in a safe. The other computer is a four core Intel with its built in conventional hard drive.
I AM STUMPED! Does anyone have a solution?