It could be your antivirus. Try disabling it’s firewall, it could be preventing Roblox from connecting to the local server that’s hosted on your computer.
In my experience, this happens when one of the clients from the test server does not get disconnected from the already shut down server and as a result prevents any new server from being launched properly.
Can you give a littlebit more information about this issue and the steps on how to replicate the issue?
I have made a post about this before but haven’t had any updates about it ever since.
Issue seems to have resolved itself, to reproduce I just started a studio test and it would consistently happen (I should have specified this in my OP, my bad).