Hi there,
yes, it certainly does look like a SuperCollider error. Unfortunately there’s not much I can do other than point you at their issues page to share the issue with them directly:
One thing you could do is try their latest beta release (3.10.3-rc1) and see if you can boot the server there:
If that works, then things should be fixed with the next release of Sonic Pi which I am planning to ship with this latest version of SuperCollider.