[quote=470300:@Tobias Bussmann]this is an old Bug in the Xojo PostgreSQLDatabase Plugin. You cannot schema-specify the table name when using RecordSet.Edit / .Update or now RowSet.EditRow / .SaveRow - see <https://xojo.com/issue/11597>
Workaround is to set your searchpath in a way you don’t need to specify a schema. This does not work for cross-schema queries with ambiguous table names - but such queries would likely not be updatable using the RecordSet.Edit interface anyhow…[/quote]
Thanks al lot for your answers. I never would have guessed it was a software bug of the PG Plugin 
I moved all tables to the public schema and it works fine.