by corvus albus.
O.k. found it: there must be an empty subfield-entry for the "text":
https:///webservice/rest/server.php?
wstoken=&
wsfunction=mod_data_update_entry&
moodlewsrestformat=json&
entryid=&
data[0][fieldid]=&
data[0][subfield]=&
data[0][value]="
data[1][fieldid]=&
data[1][subfield]=content1&
data[1][value]=1
itemid subfield is out of reach for me for now, but you can leave it out.
https:///webservice/rest/server.php?
wstoken=&
wsfunction=mod_data_update_entry&
moodlewsrestformat=json&
entryid=&
data[0][fieldid]=&
data[0][subfield]=&
data[0][value]="
html-text
"&data[1][fieldid]=&
data[1][subfield]=content1&
data[1][value]=1
itemid subfield is out of reach for me for now, but you can leave it out.