In my Webasto Next I did’t see the “system maintenance” option in the top-right menu, however this was solved just bij using another browser. 
I need to update the firmware because I want to manage the Webasto via EVCC, for which I need to enable Modbus.
Modbus does not work; I think it is because of the older firmware version.
My firmware: HMI Software Version: v2.99.11
I am trying to contact Webasto. In the mean time it would be nice is someone can point me a firmware download. Cheers!