This issue blocked me for about two days and prevented me from collaborating with another engineer at gumyum co-operative video game studio.
The other engineer's environment was Windows and he was able to interact with collab and our shared project owned by our shared org. My environment was Fedora 30 and was not working with collab, however I was able to download the project files.
The issue?
Apparently Unity naively assumes that a Linux system's copy of the root CA certificates is located /etc/ssl/certs/ca-certificates.crt. This is the default location on Debian and Ubuntu based systems but not Fedora or Redhat, which uses the /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem file.
The work around is simple, create a soft link between the paths:
sudo ln -s /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem /etc/ssl/certs/ca-certificates.crt
Please come back again soon to learn more about the games we are building over here at gumyum!