Hi,
you need to check the state s7.1.DBs.DB1._2_1 for it's type. So click on the pencil next to the state in the object tree (the pencil only appears with enabled expert mode!). There you can find under "object data" (Objektdaten in German, idk the exact translation) the type in the json. So if there is written "string" then ioBroker is expecting a string, but the API may deliver a boolean, which is then just entered into this field OR vice versa (type = boolean, API sends string). Hope this helps :)