One way to solve this issue!
I had the exact same issue but only with some files, other worked well.
Then I realized that the files that worked had no accented characters in their file path, e.g. Compositions/Concerto/Short score.notion, while the files that hanged on Connecting to Studio One had accented characters in their name or path, e.g. Compositions/Opéra 1ère partie.notion (yes I'm French).
Removing all accents solved the issue instantly, it works like a charm. So it's basically an encoding issue.
Forget ü ä ö õ á ç and such, they mess up with the connection!
Basic info: I'm on Windows 10 and use Studio One & Notion 6 with huge VE Pro templates.