I have a solution with a huge amount of projects. This itself seems to be okay, but one project has several thousand files (TypeScript, HTML, CSS, JS). When I open the solution I get a wall of fancy errors, all leading back to "Too many open files". I am only able to open the solution if I unload the project, restart Rider and reload the "special" project after a couple of minutes. Otherwise nothing works, not even syntax highlighting.
I have modified macOS High Sierra to allow several million (!) open files - makes no difference. I can remember that this dates back to the earliest betas of Rider.
It seems that Rider is trying to process too many file at once.