cant restore nuget packege on ubuntu after upgrae to .net core 3.1

i am running .net core 2.1 on Ubuntu 18.04 so far so good

i upgrade my project to .net core 3.1 and installed the 3.1 SDK

since than i cant use ‘dotnet restore’

this is the full output

/usr/share/dotnet/sdk/3.1.401/NuGet.targets(128,5): error : Unable to load the service index for source https://api.nuget.org/v3/index.json. [/repos/myproj/portal/PortalApi/PortalApi.csproj]
/usr/share/dotnet/sdk/3.1.401/NuGet.targets(128,5): error :   The SSL connection could not be established, see inner exception. [/repos/myproj/portal/PortalApi/PortalApi.csproj]
/usr/share/dotnet/sdk/3.1.401/NuGet.targets(128,5): error :   The remote certificate is invalid according to the validation procedure. [/repos/myproj/portal/PortalApi/PortalApi.csproj]

any idea why ?

thanks