I have already seen the topics about the known bug with the backpack and I am opening a new topic because the others are closed.
FYI, I have tried all your suggestions, even clicking the right button "Paste All Blocks from BackPack(n)". Clicking the right button, I can see that I have 14 blocks into my backpack, but when I select that option only 4 are copied.
Also, if I click my backpack nothing happens, although I make the steps you have suggested in the other topics.
I do not know how to copy my blocks into another project. Please, could you help me.
Same problem happens with Windows 10.
Thanks in advance.