I noticed that there was an update to the web interface so I put the recent one on github:
(Page: sonnenBatterie API )
Could you try the following? It seems to do the trick for me, no more messages:
OLD:
value_template: '{{ value_json.sonnenapiv2 }}'
NEW:
value_template: '{{ value_json.sonnenapiv2 | default }}'
Reference: