Shared ToDo deleted tasks > how to restore?

Hello everyone,

We share a ToDo with my colleagues and I deleted some tasks by mistake. I tried to restore them but whitout success.

I cannot find any place where those deleted items went. I tried a lot a of solutions found on the net but none of them worked. I also point out that those tasks were not mine.

So I am looking for some help. Does anyone have an idea or even a solution about that?