Well, I've found the issue with #IceDrive. And lost 2hours of my evening.

Thankfully, they're not the cloud system I've paid for. I was considering it when they had discounted lifetime options. But after this experience, and them clearly not being able to understand my issues, I'm glad they're a secondary service.

I haven't "fixed" the issue because I'm not a software dev. But I have a workaround that proves where the issue is.