Endless Reboot Cycle Due to Attempted OS Upgrade

I am running High Sierra on my 2015 MacBook Pro. I was having trouble connecting to VPN, so I attempted to reboot. I keep getting this alert after I type my credentials. Same alert if I restart. I never knowingly approved Mojave installation. Any ideas?

I got this too, but when I was doing a security (or maybe Safari) update on my 16 MacBook Pro at work.

Restarts did not fix this – I ended up booting from a clone and recopying said clone to the boot drive.

I believe I was then able to successfully reinstall the update. I’m still on High Sierra.

Hold down the “shift” key during startup in order to enter “Safe Boot” this should get you past this point.

This doens’t work with a wirless keyboard!

So I had, while installing another “restart required” update, this happen to me again.

I tried safe boot. No dice; the boot just hung (~15 minutes or so).

I tried safe boot with nothing attached to the computer. Also hung.

I needed to reboot from a clone and recopy to the boot drive.

As I’m not keen on reinstalling the OS, I wonder if I need to move to Mojave. I have a few laggard applications that may not yet be ready.

Not sure if it’s the same issue - but I’ve got a 13" 2010 Macbook which has been running High Sierra. Last night a security update occurred, and when I rebooted I got the Prohibited icon on boot. Trying to boot into safe mode didn’t work, so I’ve tried restoring from a TimeMachine backup (from a few days ago), which hasn’t fixed anything. Next step is to reinstall macOS and see what happens.