The service is freemium, so they had a limited account. Decided to pay for a premium account. And apparently can’t downgrade and get back what they once had.

I'm just guessing, but this:

> and have posted my rough mixes [...] on my SoundCloud for more then ten years

...easily implies >3h of uploads, which is over the free plan limit. If you're over that limit and stop paying, yes, it makes perfect sense that they'd threaten with deletion of some of your existing uploads.