Update: I took it apart and diagnosed the problem myself. If you own a Brunner Twinair Polaris 5, "Ec" means that it's probably low on refrigerant.
It's R290, so in theory you can charge it yourself, but there are no service ports and I'm not insane enough to braise onto an R290 system without nitrogen purging.
Love it when the manufacturer's response to "What does this undocumented error code mean?" is, to paraphrase, "Get fucked."