Hi danabanana,
Welcome to Apple Support Communities. To help troubleshoot this behavior, we recommend for you to restart the Mac in safe mode. This is a step that helps to verify your startup disk and clear out system caches. Once the Mac is in safe mode, please restart it as normal to see if you can open the email attachments. Here's how to restart a Mac in safe mode:
Start up your Mac with Apple silicon in safe mode
1. On your Mac, choose Apple menu
> Shut Down.
2. Wait for your Mac to shut down completely. A Mac is completely shut down when the screen is black and any lights (including in the Touch Bar) are off.
3. Press and hold the power button on your Mac until “Loading startup options” appears.
4. Select a volume.
5. Press and hold the Shift key, then click Continue in Safe Mode.
6. The computer restarts automatically. When the login window appears, you should see “Safe Boot” in the menu bar.
Start up your Intel-based Mac in safe mode
1. Turn on or restart your Mac, then immediately press and hold the Shift key until you see the login window.
2. Log in to your Mac.
3. You might be asked to log in again.
4. On either the first or second login window, you should see “Safe Boot” in the menu bar.
Following these steps may help and is outlined in this article: Start up your Mac in safe mode - Apple Support
Kind regards.