Hi Emmanuel, thank you for your prompt reply.

Yes, I have rights to write anywhere, I'm the administrator.
The file is not write protected.
I tried to save it in another folder but I received same error.

I also tried to copy the file in another folder and I renamed it, then I opened it with SH3D but again was unable to save it.

[Update] Right now I have created a new stupid file with just one room and I could save it normally but as soon as I added a background image (the one I have for my previous work) I received the same error.
The file is a 2,26 MB JPG image, is there any limitation for background files?