BSEG-SGTXT grayed in FB02

Hello SDNers,

After upgrading to ECC6 version, we realize that we can't modify field BSEG-SGTXT by transaction FB02 (the field is grayed).

I have checked the following points:

1) Posting Key

2) Field Status

3) Table TBAER

4) FI-SL

and all of them are correct.

Anyone know what could be happening?

Thanks in advance for your help

Best Regards,

Leo