I've had four of these Duos die on me. The most recent one suffered the same error as yours... stuck in altitude mode and showing an "ERR" message.
The first one that died was covered under warranty, the others have not been. Because I solo dive the vast majority of the time, I always wear two computers. But because they only die one at a time, I keep replacing them with the same type so that the profiles match. (Although, the computers don't generally read the same depth...)
In a recent email exchange with Dive-Rite, I asked if the newer models had been improved and the answer I received from Jess there basically said that the computers were "constantly evolving" or something to that effect.
At any rate, most recently I ended up with both of my Duo's dead. One was the altitude error I mentioned above, and then the screen on the second one simply faded away to the point I couldn't read it. Of course I changed batteries, reset etc. They're toast.
Anyway, I ended up replacing these with Apeks Quantums which are the same computer. I only have about 40 dives on one of them, and 5 on the other one. So far so good.
The Duo and Quantum and a host of others are all made by the same manufacturer. I am going to give these two Apeks the benefit of the doubt, but if these die, then I am going to something entirely different. I like the features of these computers, and the price point is right, but I do about 70 deco dives a year and need to be able to rely on them.
I think like so many other things these days, these are manufactured to hit a certain price point and are almost considered disposable. Unfortunately, if they die on vacation or something, it can seriously mess with your dive plan. When I am away, and diving with my wife, we use at least three computers between the two of us, so if one dies, we won't be "locked out" by the DMs...
It's not a big deal. I still think the Nitek Duo is a great computer. Just remember to change the battery if it does something goofy, before sending it back to the manufacturer.
I think that this is true. I dive in the Great Lakes and at depth, the water never gets above 41F. and could be as low as 38F. I replace batteries every spring regardless...