My 'scope is doing to same.
No ability to select AC coupling on channel A but channel B is working in both AC and DC coupled modes.
I initially thought it was some sort of software bug but hopefully reading this post has sent me in the correct direction.
I've popped the lid on the scope and can see two Hamlin relays.
One is part code HE3621A0510 (Farnell 9561927, £4.00) this is a SPST-NO relay, perhaps it has stuck closed, leaving the AC coupling capacitor permanently shorted out?
There is another relay HE721C0510 (Farnell 9561765, £3.83) this is a SPDT-CO relay.
Can anyone enlighten me as to which relay does what?
Perhaps I should replace both, unless anyone can advise otherwise?