Can't download Monterey 12.3 installer

When I try to download the Monterey 12.3 installer from the App Store, I get a dialog that says this, and just loops forever:

Finding Update

When I try to download the Monterey 12.3 installer from Software Update, I get a dialog that says his, and just loops forever:

Checking for updates …

I downloaded the Install macOS Monterey app on another Mac (no problems at all), but when I try to run it on my iMac, I get this message:

“Install macOS Monterey” cannot be opened because the

developer cannot be verified.

macOS cannot verify that this app is free from malware.

I am on a Big Sur iMac (2020, 27-inch) with more than 500GB of free space.

Help!