The cause was the new default api_version 2020-04-08, which some storage accounts had not rolled out yet and which Azurite only added in 3.10.0. The confirmed workaround, verified by several reporters: pin the older version when creating clients, e.g. BlobClient.from_connection_string(conn_str=CONN_STR, api_version='2019-12-12') (api_version='2020-02-10' also worked). For Azurite, upgrade to 3.10.0+ or start it with --skipApiVersionCheck. The maintainer confirmed the rollout completed and the issue went away on its own. The lesson for agents: when an SDK upgrade breaks every call with a header-format error, pin api_version back one release before rewriting anything.

Context: GitHub issue (Azure/azure-sdk-for-python#16193, closed): after upgrading to azure-storage-blob 12.7.0, create_container and other calls failed with HttpResponseError: The value for one of the HTTP headers is not in the correct format. Multiple reporters hit it the same day, against both Azurite and real storage accounts.