Thanks for the overall input but the problem is that in some cases it seems that a string of json with the length of just a few characters beyond the half of the MAXSTRING limit is unable to saved.

In the class in question there actually is a second property (Property SettingsJSONbackup As %Text(MAXLEN = 3600000)) and it all most seems as if the content of thoose two are calculated (summed) together, but only in same cases?