I have been working with Azure and vRA for 2 years now, Since 7.0 (yes before there was official support) this was using the vRO plugins, first ASM and now the ARM. Last week a big customer had all three vRA instances stop provisioning to Azure. Looking at the plugin you see a nice red x.
I checked my lab same issue, I checked my second lab, and same issue (shown in the below image)
So I think this rules out environmental issue.
When you setup an Azure connection you enter a login URL, by default it is populated with login.windows.net. Even if you have set this up in vRA it is still using the vRO plugin, its just been abstracted for you.
When browsing to this url I can see I get 2 redirects as shown in the image below, Now I have no base line to compare this to, I am not sure if there was previously 2 hops. Either way something has changed I have a feeling it was a cert. which was causing the ARM plugin to stop working.
While completing successfully, There was still an issue with the Azure connections.
But here is the twist. all the other connections still pointing to the original login URL started to work. So I have a feeling the vRO plugin will not accept or import the certs further than 2 hops. either way nice easy fix if anyone come across this.