Hi Guenter,
I have looked at this functionality in the past. Could you please ensure that you are setting EDID_TRIES to a non-zero value e.g. 0xC9[3:0] = 1111. Then set EDID_REREAD 0xC9[4] to 1. This should result in the EDID being updated in the EDID I2C space.
Best regards,
Joe