@kahirsch
I should also note that I also tried disabling antivirus...

@kipperchau
Thank you for the search and thought, but disabling my VPN was one of the first things I tried. I should have mentioned this in my original post.

Thanks, but no go. As I mentioned, I get the crx_file_not_readable error message each time I try to install any extension.
Any other thoughts?

I've not used Opera for a (lengthy) bit... Wanted to start again with it, and tried to install some extensions. Running an iMac, up to date. Consistently got the crx_file_not_readable error message. Rather than just resetting Opera, I deleted Opera completely and installed the current version (108.0.5067.24). I still consistently get the same error, with each extension I tried.

I noted the recommendation made in the past (in this topic: arshagarwal on March 7, 2021) about changing the permissions for site, but was unable to find the correct option ("site access"):
Click on the extension icon on the sidebar
Click on the "Install Chrome Extension" extension
Scroll down till you see site access
Enable this option - "Automatically allow access on the following sites"

Any thoughts? Thank you in advance for any recommendations.