[Solved] Can't use custom NuGet feed after updating to the TeamCity version 2017.1.5

Since the TeamCity update, all the builds that use a custom feed are failing with the following message:

CredentialProvider.TeamCity: Failed to find TEAMCITY_NUGET_FEEDS var in environment

Please provide credentials for: {feed here}

What is weird is that in the build log it actually seems like it was trying to use the user that is stated in our nuget.config, as I can see this message just before it fails:

Using credentials from config. UserName: user@domain.com

That user is defined in our nuget.config file in a following format:

<packageSourceCredentials>
  <customFeed>
    <add key="Username" value="user@domain.com" />
    <add key="ClearTextPassword" value="Password" />
  </customFeed>
</packageSourceCredentials>
<packageSources>
  <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
  <add key="customFeed" value="custom/feed/here.json" />
</packageSources>

The user obviously still works when building the project elsewhere.

EDIT: Never mind, it was an issue with the custom feed after all and not TeamCity.