The reason you need to press "BACK" is that you are no longer on the forum page that you were viewing. You are on the page that is displaying the message. You can press "BACK" or you can press the X in the top corner of the error message. Either will put you back on the forum page, which you can now refresh. If you refresh before you get rid of the error message, you are simply refreshing the error message. You can provoke a similar error message by attempting to upload an image with a disallowed character in the name, such as an apostrophe.
I've opened a tab for another Xenforo forum on my MacBookAir, which I will now leave for a couple of days. When I go back to it, I'll see if the behaviour can be provoked. The behaviour is a feature of Xenforo, not one browser or another, but for the record, MacBookAir 15", M3, 2024, running Tahoe 26.5, using Safari.