Why AT+WOPEN=6,X returns CME ERROR:515 for fw 7.51 & above?

Please note that for 7.51 and above, it should be noted that the A&D service is emulated above the file system. If the File System area size is not null, any resizing of the A&D area is impossible. So the File System area size must be set to 0 (with AT+WOPEN=9,0 command) before resizing A&D using AT+WOPEN=6,XXX command.