I have a file created with QCAD 2.2 that I have tried with 3.0B3 and it opened fine. I did not test there extensively.
I just downloaded 3.0RC1, and when I open the file some parts of a block I use from the Library (an A3 template) don't scale correctly, while other parts do.
I was "just learning" blocks when I made the edits, so may have done something incorrectly, but 2.2 and 3.0B3 were OK with it.
If you want to look at the file, let me know.