If you need me to delete the post, let me know, but I found the solution here - which can be silly - but for me it wasn't...
What happened is that C3 at the time of saving the buckap file was not automatically saving as .C3p as I was used to in C2.
I needed to rename the local file and put the extension manually and it opened.
Now, every time I save a file in C3 I have to put "filename.C3p" and then it will open normally... I believed that C3 did this by itself
grateful to everyone.