the computer where I edit HTML files has Windows XP installed, the other two are with Windows 7 64 bit.
Notepad++ is quite old, v5.5.1
I don't think this issue is related to Notepad++. I noticed this first with "Evrsoft 1st Page" editor, but thought it was related to Evrsoft, because this program is not so stable. However, I am using it for a long time and got corrupted files only after I started to sync my computers with Dropbox+Boxcryptor.
When I noticed the same issue with Notepad++, I wrote the message here.
I just sent the debug log to you.