I'm unable to edit certain WiFi connections, even though I added and
successfully edited them before. I can change whatever I like in
nm-connection-editor, but the 'Save' button is greyed out.

The connection in question is a "WPA & WPA2 Enterprise" with PEAP and MSCHAPv2
authentication. If I call nm-connection-editor from the command line, I see a
lot of warnings:

(nm-connection-editor:3364): Gtk-WARNING **: Failed to set text from markup due
to error parsing markup: Error on line 1: Entity did not end with a semicolon;
most likely you used an ampersand character without intending to start an
entity - escape ampersand as &

When I edit the connection in question, I see:

** (nm-connection-editor:3364): WARNING **: Invalid setting Wi-Fi Security:
Invalid Wi-Fi security

The same error is shown every time I change the value of the "Security"
dropdown, regardless of what I select.

The only workaround is to create a new connection