M400, cannot change system chain ID

Hi everyone, I'm on the M400 webinterface. I want to change the system chain ID. There is a longer number in the field, so somebody could write something into the field, at one point. However, neither can I add someting, nor delete something from the system chain ID field. The mouse cursor is in the field, when I click it but that's it. I restarted the M400, reset it, and nothing helped. I also had manually edited the config file and uploaded the edited config file. No success. The number remains the same unedited one, from before uploading the edited config file. 
I'm out of ideas. What is the reason for this behaviour? 

Please help.