Can't edit subscription settings. Doesn't appear to be Wordfence/mod security issue

I’m in Config>Settings>Subscription Settings here:

URL where subscribers can sign up

When I make edits, they appear to be accepted–but the URL doesn’t actually change.

I read forum posts advising it might be one of two issues:

*Wordfence Plugin - But I have this deactivated
*Mod security - But this doesn’t appear in either htaccess file on my server

Is there a way I can edit this URL via FTP?

Or is there something else that could be blocking these updates?

By default, this url uses the server name. I believe that when first setting up phpList, it plugs the servername into the subscribe url. If you later change the servername, the subscribe link is not updated… you have to do it manually.
The url is saved in the phpList database, so if all else fails, you could change it there using an sql statement and phpMyAdmin or equivalent.