I am sorry, but it seems to be caused by something locally on your side.
I cannot reproduce the issue on your site either.
I logged in, opened a Content Templater item, clicked on 'Save & close', and all is fine. It returns me to the list (not the control panel).
I recommend you try this on a different browser. And see if any of your browser plugins is causing issues.
So here's a bit more info:
* All I am running locally is windows security and Malwarebytes (I have whitelisted this domain).
* I tried turning off RS Firewall (no effect)
* I have no browser extensions installed and I tried this is Chrome, Brave, FF, and Edge on three different computers.
* Also disabled my wired network (comcast business) and use my verizon hot spot. Same effect.
These issues are so frustrating. I've been here too many times.
I did one more test on my laptop, which is a generic windows 10 home computer. No plugins, windows security.
I also rolled back JCE one version, same effect.
Something is going on and I'm guessing this will start being reported as the module is updated and installed.
I think I tested all the possible variations (computers, browsers, internet providers). As a final check, I tested it on my 10 year old iMac 21 (with up to date browsers and OS) in Brave and Chrome and it got the same effect: 9 works, 10 does not.
Note: If you want me to, I can clone the site and give you full access and CPANEL access.
So for now I'll stay on 9. Let me know if you find a solution.