I have had similar issues with a proxy on a web-site, and have installed the updated proxy. This seems to have removed the first error. However, now I am having similar issues to Francesco with the authentication as well.
My proxy authenticates the AGOL credentials (using a username & password), and this seems to work perfectly well.
The proxy also authenticates ArcGIS Server services that are referenced in AGOL as map image layers. The credentials are not stored with the map image layer, but the proxy is meant to deal with this. And it has been doing this successfully until the last week. Not exactly sure when it stopped working, but I assume this is tied in with the TLS issue.
Our ArcGIS Server is still 10.5.1
Does our server need something installed?
Thanks
Keith