Good to hear!! Yeah, I will have to do some thinking on this.
The RSPM repository does not have packages pre-downloaded (lest the QuickStart be 200+ GB
). As a result, it still has to go fetch the packages when you request them. To do that, it reaches out to the URL that you saw in the error log: https://rspm-sync.rstudio.com . Because that is an SSL domain, it has the same certificate problem that you saw when going directly to CRAN.
There is a proxy or something between you and the internet that is doing some SSL certificate shenanigans and your computer doesn't like it. This would not be surprising in some corporate environments, which I suspect is your case, although perhaps you can confirm.
These types of SSL / certificate errors are worth paying attention to generally, though, because it can also evidence a Man-In-The-Middle (MITM) attack, where someone/something nefarious is trying to decrypt or intercept your traffic.