Encrypted TimeMachine - having bad times "make sure it's not mounted by another machine and try again"

Well, hi all. I hope you're good!

So, I have a Mac Book Pro 13'' and a iMac. And also have TimeMachine in my FreeNAS. So, I had bad time with long... looong backups with TMC and had seek all over the internet to solve it. So far so good.

Today, I had an issue with my iMac and tried to send my back up to my MacBook. But I seems impossible to do so. I've also tried everything I found in the internet and what I could think of. Nothing solve it!

This TMC back up is Encrypted with the TMC's option of encryption. So, I thought it was safe... But no. See the pic bellow. I've first tried to recover from freeNAS. But It couldn’t. So I've copied the files in the dataset to an external WD HDD. And NO LUCK again, look there, the very same error again (I thought was a freeNAS issue...). So what are my changes? I've lost everything? There's no way to recover it? no even my personal data?

What is so hard to make things easy?