After upgrading to ArcGIS Pro 2.8 our Jupyter notebook scripts handling content migration have broken.
They are no longer able to authenticate against our proxy and are reporting the following issue:
ConnectionError: A connection error has occurred: HTTPSConnectionPool(host='[DOMAIN].gov.au', port=443): Max retries exceeded with url: /portal/sharing/rest/info?f=json (Caused by ProxyError('Cannot connect to proxy.', OSError(0, 'Error')))
I feel the upgrade has broken some of the python code handling proxy_host.
Update: ESRI confirm it's a bug (BUG-000140337). Only solution is to roll-back Pro to 2.7 until a fix is provided. It might be related to BUG-000140216, which is covering the core Python API.
I have encountered the same problem. In my estimation, the bug is related to the newer versions of the urllib3-module. There, the proxy call must be made using the ProxyManager function. I escalated the bug because otherwise ArcGIS Pro version 2.8 won't work for us.