Luc, I think the issue might be due the OS which you are on.
As the OS 10.11 El Capitan is not officially released that can be the reason its not working.
Can you try rolling back to older OS and confirm how that works.
I tested your solution on my MacBook pro running Yosemite and it worked.
Yet, this doesn't explain why on my iMac I can use 5.7 but not 6.1...
The Imac on which you are using 6.1 can have any compatibility issues as the official release is not out.
I would suggest to wait for the final release of OS as well as team might fix the issue for Lightroom also with the updates which may be released in future.
You haven't explained what happens when you try to import using Lightroom 6. So please explain. What do you see? What happens? What is the exact word-for-word verbatim error message?
For future reference, you would get more speedy answers if the error message was stated in the first message in this thread.
This issue has nothing to do with operating system or version of LR or camera used. It means that the destination folder (as specified in the Import Dialog box, destination section on the right hand side) does not have write permission. You must go to your operating system and change the permissions on this EXACT folder to WRITE permission.
In fact, a "/" and then blank destination had appeared in the "recent list". I just had to choose to the proper destination, clear the recent destination list and now all is just fine.
Yeah, thanks for the hint...