Apologies for the late reply! As you suggested, I downloaded the latest build which shows up as Version 1.6.1 (2021-04-23). Unfortunately, it does not solve my problem as adding Plots still results in an error:
(@v1.6) pkg> add Plots
Updating registry at `C:\Users\me\.julia\registries\General`
Updating git-repo `https://github.com/JuliaRegistries/General.git`
Resolving package versions...
Downloaded artifact: Libgpg_error
Downloaded artifact: Libgpg_error
ERROR: Unable to automatically install 'Libgpg_error' from 'C:\Users\me\.julia\packages\Libgpg_error_jll\3LF6G\Artifacts.toml'
Stacktrace:
[1] error(s::String)
@ Base .\error.jl:33
[2] ensure_artifact_installed(name::String, meta::Dict{String, Any}, artifacts_toml::String; platform::Base.BinaryPlatforms.Platform, verbose::Bool, quiet_download::Bool, io::Base.TTY)
@ Pkg.Artifacts C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\Artifacts.jl:443
[3] ensure_all_artifacts_installed(artifacts_toml::String; platform::Base.BinaryPlatforms.Platform, pkg_uuid::Nothing, include_lazy::Bool, verbose::Bool, quiet_download::Bool, io::Base.TTY)
@ Pkg.Artifacts C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\Artifacts.jl:508
[4] download_artifacts(ctx::Pkg.Types.Context, pkg_roots::Vector{String}; platform::Base.BinaryPlatforms.Platform, verbose::Bool, io::Base.TTY)
@ Pkg.Operations C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\Operations.jl:706
[5] download_artifacts(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; platform::Base.BinaryPlatforms.Platform, julia_version::VersionNumber, verbose::Bool, io::Base.TTY)
@ Pkg.Operations C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\Operations.jl:683
[6] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}, new_git::Vector{Base.UUID}; preserve::Pkg.Types.PreserveLevel, platform::Base.BinaryPlatforms.Platform)
@ Pkg.Operations C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\Operations.jl:1237
[7] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; preserve::Pkg.Types.PreserveLevel, platform::Base.BinaryPlatforms.Platform, kwargs::Base.Iterators.Pairs{Symbol, Base.TTY, Tuple{Symbol}, NamedTuple{(:io,), Tuple{Base.TTY}}})
@ Pkg.API C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\API.jl:203
[8] add(pkgs::Vector{Pkg.Types.PackageSpec}; io::Base.TTY, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\API.jl:79
[9] add(pkgs::Vector{Pkg.Types.PackageSpec})
@ Pkg.API C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\API.jl:77
[10] do_cmd!(command::Pkg.REPLMode.Command, repl::REPL.LineEditREPL)
@ Pkg.REPLMode C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\REPLMode\REPLMode.jl:408
[11] do_cmd(repl::REPL.LineEditREPL, input::String; do_rethrow::Bool)
@ Pkg.REPLMode C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\REPLMode\REPLMode.jl:386
[12] do_cmd
@ C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\REPLMode\REPLMode.jl:377 [inlined]
[13] (::Pkg.REPLMode.var"#24#27"{REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::IOBuffer, ok::Bool)
@ Pkg.REPLMode C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\Pkg\src\REPLMode\REPLMode.jl:550
[14] #invokelatest#2
@ .\essentials.jl:708 [inlined]
[15] invokelatest
@ .\essentials.jl:706 [inlined]
[16] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
@ REPL.LineEdit C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\LineEdit.jl:2441
[17] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
@ REPL C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\REPL\src\REPL.jl:1126
[18] (::REPL.var"#44#49"{REPL.LineEditREPL, REPL.REPLBackendRef})()
@ REPL .\task.jl:411
julia>
The Downloads.download() also still gives the same error. I tried various workarounds to add the proxy certificate to my machine, but this exceeds both my knowledge as well as my administrative rights (corporate environment). So rather than bug IT about this, I read through the thread on issue #108 that you linked, and after some experimentation, I can get it to work by going the insecure route of ENV["JULIA_SSL_NO_VERIFY_HOSTS"] = "**".
This is obviously not ideal but until Julia becomes a standard install at work I think may have to run it like this.
Thank you again for your invaluable help!