When using Codex with GPT5.5, I saw "Selected model is at capacity. Please try a different model."

This has never happened to me with Codex before.

So this means they've begun re-allocating the serving provisions. They've switched a large portion of 5.5 serving capacity over to 5.6 in anticipation of imminently changed load.

5.6 for Thursday was already officially announced.

As to the capacity issue: no, it's happened before. Maybe related. Maybe not.

Yes it is.