"Endotheliitis is an immune response within the endothelium in blood vessels, in which they become inflamed. The condition can cause oedema of the surrounding tissue, including the stroma, and can cause irritation and pain. If it is within the cornea, it can result in permanent loss of vision. The condition can be caused by a number of factors, such as mumps and cytomegalovirus under certain circumstances." (Wikipedia)

And, as to the first question that came to my mind, from the paper: "Collectively, our results suggest that the S protein-exerted EC damage overrides the decreased virus infectivity. This conclusion suggests that vaccination-generated antibody and/or exogenous antibody against S protein not only protects the host from SARS-CoV-2 infectivity but also inhibits S protein-imposed endothelial injury and ultimately decrease cardiovascular complication-associated mortality in COVID-19 patients."