If that is a version by the original authors, it's likely extremely old. The version I started from last year was 6.14.4. I had to do quite a bit of work to get AMXBans to work with a supported version of PHP (version 8.1+) so I can't imagine that you're using a safe version of PHP.
In my PHP8 updated version, I was able to add a server admin with the name "!, @, #, $, %, ^.. etc." (literally just copied and pasted that from your post into the "Name" input. I did the same thing for adding a web admin and it worked perfectly fine as well.
You can get my PHP8 upgrade (from version 6.14.4) on my GitHub
here.
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