Whenever I try to save a change I made in Regularlabs ReReplacer component I get a 404 error, it only happens on one site. How can I resolve this issue?
So this definitely looks like an issue caused by something on that setup, not something inside ReReplacer itself.
Maybe some firewall extension getting in the way, like RS Firewall or AdminTools?
I recommend you check the PHP error logs to see if you find anything that indicates what is happening. Or ask your host.
First probably a good idea to reinstall ReReplacer (latest version).
And maybe try disabling other 3rd party extensions (plugins) which you think could cause this...