[54000] ERROR: Cannot prepare statement - too many prepared statements

Hi,
at the risk to ask this question again, but I could not find a solution that suits me.
I'm using the Vertica.Data 7.2.3. nuget package to insert many data sets into my database. The code for a single row is basically something like:
string cols = string.Join(", ", keys.ToArray());
string values = string.Join(", ", data.Values.ToArray());
_command.CommandText = "INSERT INTO " + table + "(" + cols + ") SELECT " + values;
affectedRows = _command.ExecuteNonQuery();
This snippet runs fine for a couple of hundred rows, but then I get
[54000] ERROR: Cannot prepare statement - too many prepared statements
Where do I 'prepare statements' and how can I reset? It looks like I need to reset the 'prepared statement' buffer or soemthing like that.
I also tried a commit after each insert, but then I got a message that the statement was alread commited.
Thanks , Joerg