Just registred to thank all you guys. It works!!!!
- Root the phone;
- Delete all files in /data/clipboard;
- Reboot.
That's all I did. Clipboard working fine!!!
SGS3 is a hell of a phone but Samsung is messing things up by not solving this problem untill now.
Good luck!
ps.: It seems to me all started after I used S-Voice for the first time...