This copy of the MacBook Pro Install macOS Monterey application is damaged

Interesting follow up to this, and thanks to @North Hills Guy for mentioning F-secure. I was unable to upgrade to Monterey on my work Mac, a 2019 16" MacBook Pro (Intel Core i9, 16 GB RAM, 1 TB SSD). I received either the 'installer is damaged' error or the 'an error occurred loading the update' error, always about 2 minutes into the install. Trashing and re-downloading the installer did not help. Doing that then booting in Safe Mode and downloading/running the installer did not help. Did not matter if I was on WiFi or Ethernet (via a Stone Pro dock). Booting in Internet Recovery to install the latest OS (Command-Option-R at boot) also failed ('an error occurred loading the update').

My personal Mac is a twin to my work Mac, and had no issues upgrading to Monterey. Nor did my wife's 2020 13" MBP, and I was able to update my kids' MacBook Airs via Internet Recovery (which I did to save having to remove 12 GB worth of data to free up space). Just my work Mac gave this issue.

Although I don't have F-secure, we do use Forticlient for VPN access to the company intranet from offsite. Despite using the VPN Only installer for Forticlient, I did see the occasional requests for permissions that North Hills Guy mentioned, which I had also denied each time.

Long story short, simply quitting the Forticlient app and running the installer did the trick. Honestly, I found this surprising since neither Safe Mode nor even Internet Recovery, neither of which should be affected by a 3rd party app running in the background, enabled me to run the update.

Regardless, I've now got Monterey running successfully on all my Macs.