Mac username & password for proxy settings keep resetting by itself
This worked for me. Here's what I did:

The solution is to ensure that you have working proxy settings saved in the system keychain, not just your personal login keychain.
So when you next get one of these dialogs:

Note the server name that is requesting authentication, the port (usually 8080) and if it is an http or https request. Typically it will be something in the form https://someproxyserver.someorganization.com:8080

See if you already have an entry for that server in your login keychain. If not, make one manually, (being sure to enter the whole thing as per the example above with the :8080 at the end). Typically that there needs to be two separate keychain entries per proxy server, one for http and a second for https, though syncdefaultsd only uses https.

Once you have login keychain entries for the proxy server, double-click them and ensure that under Access all applications are allowed, and that your user name is saved.

Now for the fun bit. Option-drag and drop these entries into your system keychain. Click on the system keychain, and confirm that they are there, and that all the settings are exactly right.


For good measure, do a shift-restart then a normal restart.
You should now get no more annoying dialogs for that particular proxy server. If your proxy server has more than one alias, or if you have several, then whenever you get a new unidentified dialog, repeat the above.

Source: Apple Support Discussions thread