No Line-Out on K9

Hi all,

I've an issue with the Line-In -> Line Out path on my Fiio K9. Situation is as follows.

BT -> Line-out (single ended): OK

USB -> Line-out (single ended): OK

Line-in (single ended) -> Headphone (6.3mm): OK

Line-in (single ended) -> Line-out (single ended): FAIL - No sound

I've tried multiple sources on the Line-In, no success. There is no other input connected.

Any suggestions? Is my device broken or am I not using it propperly?

Thanks for your kind help!