Could the June update be causing a black screen after login?

I’d treat the update as a lead, not a confirmed cause yet. Reports of similar symptoms include systems on an earlier Windows version without the June update, and some machines reportedly aren’t domain joined. That makes a single patch explanation less certain.

Build a small comparison table: affected and unaffected machines, update level, boot type, service state, security and backup software, and the exact time of the failed login. Compare event logs around that time, especially the reported Winlogon notifications, rather than relying on general boot performance results. If the problem follows one update or configuration in a controlled test, that gives you a much stronger case.

SessionEnv sounds worth testing, but the mixed results argue against treating it as the fix. Try changing it only on a few affected machines, record the original setting, and see whether the symptom stays away across several cold boots. If it returns while the service is running, that’s useful evidence too.

I also wouldn’t create a LowLatency registry value just because it’s missing on an affected PC. First confirm the relevant policy and setting behavior for this Windows build, then test any change on a small group with a clear way to undo it.

If Task Manager opens, use that window to check whether Explorer is responsive, but don’t assume restarting it proves the shell is the cause. A logon can stall elsewhere even when Explorer is running.

I’d also compare endpoint-security and storage-filter components on affected versus unaffected PCs. A clean system file scan doesn’t rule out a driver or service delaying logon. Test changes narrowly, and avoid broadly disabling protection just to see what happens.