Re-posting in case people missed it before. Can someone help me get to the bottom of this?

Could you try and capture the raw network request? It should get logged with the following to stderr:

pulumi up --yes --skip-preview -v=9 --debug --logtostderr --logflow

you might need to filter out a bunch to isolate the request but that might be the closest means to identify what is wrong. Feel free to open an issue with the details if there is nothing sensitive there. Otherwise feel free to DM me a redacted version and I can investigate after opening an issue.