When you start LR, a lock file is created next to the catalog to prevent others to access it.
The lock file is deleted when you close LR, but if it's not closed down properly, the file will not be deleted.
So delete the file called 2015-09-27 Marco+Elena.lrcat.lock, and the catalog should open.
It may also be a good idea not to run the export process and create previews at the same time. Sounds like a lot of work for a computer with limited power.
Note that grey boxes like those in the second image above often indicate a corrupt monitor profile.
What color should they be? Where can I investigate more about this thing? It's totally new to me. Never heard. Thank you.
The OS doesn't let me delete that file, neither the .lrcat-journal one. Because "some data in that file cannot be read or written (error code -36)". Well this is just a translation from the italian error message, actually.
some data in that file cannot be read or written (error code -36)
I don't use a Mac, so I can't help you with that, but a Google search for "error code 36 mac" gives plenty of reults.
Keith_Reeder is referring to the missing thumbnails in the second image that you posted, which could indicate a broken monitor profile.
I let LR export a couple of thousand photos overnight, while I asked it to generate the standard previews of the same files. The export reason was to send the previews to my client before I started working on the whole project
Just to make sure you are getting this right - previews and exported files have nothing to do with each other.
The previews are for internal use in Lightroom only. Since LR doesn't touch the original images, it has to build previews.
Simple fact is if you are getting a corrupted LR catalog file more than once you have something wrong with your computer. Something in your computer is corrupting the catalog file. You have faulty hardware. That could be a faulty hard drive or bad RAM.
I have been using LR since Versions 1 became available, along with many other programs from Office programs to music and CAD programs on many different computers both Windows and Mac OS X and never had a corrupted LR catalog file or any file that I can remember. But then I have had properly working computers with solid functioning hardware. The few times I have had any problems with systems not booting or crashing was because of faulty hardware. Once that hardware was replaced I've never had any problems.
So start investigating the hardware in your Mac.