This just tripped me up again!
A slight variation on the theme, but its clarly exactly the same issue...
I had resolution set to "Auto resolution", and I was already using channel A & B for other stuff, so I used channel C as my trigger channel. All was well.
I attached the external trigger probe to the same signal as channel C, and changed my trigger source to "External". All was still well, triggering level set nicely.
Now that everything was pretty much as I wanted, I disabled channel C, but oh no - triggering stopped.
After about half an hour (slightly embarassed about how long this took me) I realised that changing from 2 to 3 channels set the auto resolution to 14 bits, so the external trigger worked ok. When I disabled channel C the auto resolution went to 15 bits, and everything stopped because of the issue described in the original post. If I enable channel D (which I'm not using currently) then triggering starts working again. Obviously if I change the resolution to 14 bit then it works also.