Credential provider returned an invalid result

Reproduce the setup with TFS, the Paket credential manager, and a paket.dependencies file containing only a NuGet v3 source. Start from the logged credential-provider failure and compare it with the behavior when the TFS source is added; done means identifying whether Paket or the credential manager is responsible and resolving the failing setup.

Description

I'm not quite sure if this is a bug in paket or the credential manager. In the following setup

1. TFS with paket credential manager
2. paket.dependencies with only nuget v3 source

Paket seems to download all packages but fails in the end and prints:

2018-07-12T14:27:58.0303043Z   Possible Performance degradation, V3 was not working: One or more errors occurred. (One or more errors occurred. (Credential provider returned an invalid result (0): 
2018-07-12T14:27:58.0312809Z    Standard Error: ))
// more logs

2018-07-12T14:28:02.0939369Z   There was a problem while setting up the environment:
2018-07-12T14:28:02.0958901Z   -> One or more errors occurred. (Credential provider returned an invalid result (0): 
2018-07-12T14:28:02.0958901Z       Standard Error: )
2018-07-12T14:28:02.2248013Z   -> Credential provider returned an invalid result (0): 
2018-07-12T14:28:02.2248013Z       Standard Error: 

If you add another source with the TFS url Paket prints (but works in the end):

2018-07-13T06:58:03.0658053Z   Possible Performance degradation, V3 was not working: One or more errors occurred. (could not get registration data from https://api.nuget.org/v3/registration3-gz-semver2/runtime.native.system/index.json)
2018-07-13T06:58:03.3158056Z   Possible Performance degradation, V3 was not working: One or more errors occurred. (could not get registration data from https://api.nuget.org/v3/registration3-gz-semver2/system.reactive.compatibility/index.json)
2018-07-13T06:58:03.3314313Z   Possible Performance degradation, V3 was not working: One or more errors occurred. (could not get registration data from https://api.nuget.org/v3/registration3-gz-semver2/system.security.cryptography.openssl/index.json)
2018-07-13T06:58:03.3314313Z   Possible Performance degradation, V3 was not working: One or more errors occurred. (could not get registration data from https://api.nuget.org/v3/registration3-gz-semver2/fake.core.environment/index.json)
2018-07-13T06:58:03.3626770Z   Possible Performance degradation, V3 was not working: One or more errors occurred. (could not get registration data from https://api.nuget.org/v3/registration3-gz-semver2/system.memory/index.json)

Interestingly the Urls are valid and seem to respond with data.