Bad Request: Post Request Required when saving hyperlink in specific redactor field
From 'p' =&gt; '500.shtml' it looks like your server is configured to redirect to a 500.shtml file whenever it encounters an error, which is probably confusing Craft routing and obscuring the real error message.

I'd check to see if you have Suhosin or ModSecurity installed on the server as it's possible the link you're trying to add it tripping up one of their rules.