I spoke with gabrielks12 on Steam about this, but for the benefit of other users:
My best guess is that Steam Community Data is not accepting a connection from his web server for some reason, since there is not a response no matter what profile URL is tried, and Steam Community clearly isn't down. It does not appear to be a configuration issue (certainly nothing related to the plugin's configuration could be causing it). I haven't yet determined what is causing the connection to be rejected. If anyone else is experiencing an issue where the above error is shown whenever any user tries to add their Steam info, and the issue isn't temporary (it may show up temporarily if Steam Community is down), let me know.
My best guess is that Steam Community Data is not accepting a connection from his web server for some reason, since there is not a response no matter what profile URL is tried, and Steam Community clearly isn't down. It does not appear to be a configuration issue (certainly nothing related to the plugin's configuration could be causing it). I haven't yet determined what is causing the connection to be rejected. If anyone else is experiencing an issue where the above error is shown whenever any user tries to add their Steam info, and the issue isn't temporary (it may show up temporarily if Steam Community is down), let me know.