Successful file transfer but file content is wiped out!

Hello!
I've been using FileZilla client to FTP files to my web server for 5 years without any problem. Suddenly, yesterday when I moved my file as usual to the server I got prompted for "This file already exist" 2 or 3 times. Usually I only get 1 warning.
And the worst thing is, the codes inside my file was wiped out completely.
I tried it again and again to a different folders this morning and I still got the same result, content of whatever file I moved to my server got wiped out completely!

I checked for folder permission, nothing got changed, so what is going on? does anyone ever gets this problem?

Thanks