In 7.5 we reworked quality gates, including the drop of Warning status. The Upgrade procedure tells users to re-analyze all projects (immediately) post-upgrade. And yet that can be an unreasonable expectation for the kind of large Enterprise instance which is most likely to make heavy use of Portfolios.
When a portfolio contains a project where the Quality Gate status is Warning (i.e. no longer valid) calculation of the portfolio fails: