The data length of the gateway cannot be set via the Sercos bus. If the controller tries to do this, it would explain error messages.
However, the data length can be set via the web server:
It should be noted that the values to be entered on the web server each contain 4 bytes of control data that are added to the user data (all length specifications in bytes). The standard data length 18 therefore includes 14 bytes of user data.
Strictly speaking, only the maximum data length is set on the web server of the Sercos gateway. If the valid one for the partner gateway is different, the lower value is used.
In connection with the EtherCAT gateway e.g. doesn't matter.