In some cases, the update causes a conflict with Microsoft Defender Advanced Threat Detection which causes it to stop running and sending reporting data. This is sometimes characterized by a 0xc0000409 error in Event Viewer under MsSense.exe.
The issue won't be fixed until an estimated mid-November, which means affected users should roll back as soon as possible.
“At this time, we suggest that devices in an affected environment do not install KB4520062,” said Microsoft.
The Microsoft Defender bug is joined by one surrounding Asian language packs. some users with them installed will get the error 0x800f0982 – PSFX_E_MATCHING_COMPONENT_NOT_FOUND. They must reinstall language packs to fix this or install the April 2019 Cumulative Update.
Finally, the build has problems with files and folders on a clustered shared volume. Users may get a bad impersonation level error when the owner node doesn't have admin rights. The only workaround for this right now is to perform it from a process that has admin privileges or from a node that doesn't have CSV ownership.
All of these are frustrating bugs, so it's fortunate that this is only an optional update. Microsoft hasn't had the best track record for stable updates of late, with recent Intel Display Driver and Start Menu issues.
A fix for the Start Menu bug will come in late October. Microsoft is busy right now prepping the November 2019 feature update, but generally we're seeing more and more reasons for users to configure their settings to delay updates by 30 days.