3.5.21 is the latest launcher version.
Spine downloads the update file and saves it in the temp directory. That is the temp8950185162262402348.tmp
file you see in the log. When the download is complete, it copies the temp file to the Spine/updates
folder in your user folder. This way the updates
folder will never contain a partially downloaded file. The error you see is because it could not find the temp file it just downloaded!
I would guess something is deleting the temp file. Your antivirus should have some sort of log so you can track which files it randomly deletes (I'm not a big fan of such software!). Try disabling your antivirus when Spine needs to download an update file. You can turn it back on afterward, but you'll want to make sure it doesn't delete the update file from your updates
folder:
Windows: <user home folder>\Spine\updates
Mac: <user home folder>/Library/Application Support/Spine/updates
Linux: <user home folder>/.spine/updates
We could change the launcher to download the update file to a path under the Spine user folder, but there are other files Spine might need to write to the temp directory. If you are running an antivirus or other software that goes around deleting files that other apps may be needing, that has the potential to break any or all of the apps you run.