When a cert is specified for a repo, the solver will use the cert and ca list when querying for packages, but after solving won't pass the cert and verify values when constructing the requests session to download the packages. There's a fix in https://github.com/python-poetry/poetry/pull/3490, but it has been pending review for several months.

There's a second issue (this one isn't cert related) where repos that return a 403 when a given package doesn't exist causes the solver to abort, which is problematic if your private repo is Artifactory, since the solver interprets this as an auth misconfiguration and won't fall back to the primary repo.

Based on some comments in the threads for those PRs I wondered if maybe the delay in merging was that some changes to the way repositories are configured, but it'd be great to have a temporary workaround for the time being.