I'm guessing this was involving a partner flight since this was flying in EU? Sounds like the partner had stale award inventory showing in UA's system, and when the website went to confirm it, the partner's system refused the booking as that inventory wasn't actually available. Proper way for this to be handled is to confirm the new space first before dropping the old space. A good phone agent who knows what they're doing can do this to avoid losing the existing space, but unfortunately the nature of a web-based system that is designed to process everything in one transaction can't handle that without also causing issues with inventory getting confirmed but never ticketed and just sitting in limbo/unavailable for other customers.

I'll still of the opinion their online experience far surpasses other airlines, even if it does fall short on some parts of award booking with partners or award ticket changes.