I just tried to set up that redirect on your account and I didn’t get any errors like that.
I see a lot of people recently reporting similar errors in some control panel sections. But any time I try it myself, it just works. I have no idea why it would work for me but not for you.
Yes, I set the redirects to the control panel, which means it went through the indexpl.php entry point.
I appreciate you’re willing to help think of possible explanations, and I do know that bad file permissions can cause 403 Forbidden errors.
But if file permissions in the control panel code would be the problem, then the entire control panel would be down for everyone. I think we’d all notice if that was the case.
Similar issues have occurred in the past due to mod_security, a Web Application Firewall implemented to protect the control panel against hacking. Some URLs or payloads can be incorrectly detected as malicious and trigger a block, which is shown as a 403 Forbidden page.
If that’s the case, we can fix it. But to fix it, we need to know which parameters cause the block.