mono paket fails sporadically with ssl exception during web requests

Description

With mono, sporadically (~ 4 %) of times, paket fails making a web request, with an ssl exception

Repro steps

using mono (repro with 5.4) doing repeated paket update

Expected behavior

paket works as expected, doing correct web requests

Actual behavior

paket fails with exception doing a web request

-> Could not find versions for package Nancy on https://www.nuget.org/api/v2.
-> Source 'https://www.nuget.org/api/v2' yielded no results
	-  Request 'https://www.nuget.org/api/v2/FindPackagesById()?semVerLevel=2.0.0&id='Nancy'' finished with error
	-> Could not retrieve data from 'https://www.nuget.org/api/v2/FindPackagesById()?semVerLevel=2.0.0&id='Nancy''
	-> HttpRequestException: An error occurred while sending the request
	-> WebException: Error: TrustFailure (One or more errors occurred.)
	-> AuthenticationException: A call to SSPI failed, see inner exception.
	-> MonoBtlsException: Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED
	     at /build/mono-5.4.1.7/external/boringssl/ssl/handshake_client.c:1132
	-  Request 'https://www.nuget.org/api/v2/Packages?semVerLevel=2.0.0&$filter=Id eq 'Nancy'' finished with error
	-> Could not retrieve data from 'https://www.nuget.org/api/v2/Packages?semVerLevel=2.0.0&$filter=tolower(Id) eq 'nancy''
	-> HttpRequestException: An error occurred while sending the request
	-> WebException: Error: TrustFailure (One or more errors occurred.)
	-> AuthenticationException: A call to SSPI failed, see inner exception.
	-> MonoBtlsException: Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED
	     at /build/mono-5.4.1.7/external/boringssl/ssl/handshake_client.c:1132

real issue is

MonoBtlsException: Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED

Known workarounds

retry.

Additional info

reference issue https://bugzilla.xamarin.com/show_bug.cgi?id=57516 in mono bug tracker

Environment

mono 5.4.1.7 on ubuntu

Start by reproducing repeated paket update requests with Mono 5.4.1.7 on Ubuntu and inspect the reported TrustFailure and MonoBtlsException. Read the referenced Mono issue 57516 to determine whether Paket has an actionable scope; done would require reliable web requests or a documented resolution.