I am also getting the problem.... this is most likely a bug with version 5.7 of the connector.
I found that if I specify the fields in the query, not use select *, AND take out all custom fields (those starting with Z), it will work. Then I can put the z fields back in, and run it again. I know this sounds crazy, I am still debugging the ABAP used for the SAP Connector to see how it's creating it's SQL statement to send to the database.
So change this:
select * from TABLEX
select c1 c2 c3 from TABLEX
now run it....
now change it to this:
select c1 c2 c3 z1 z2 .... from TABLEX
I'll keep you posted on the debug results. We may have to upgrade to 5.8. Also still investigating a possible note that would fix SAP.
it appears that my first run, which failed, was run with a packet size of 20000, even thought I had it set to 5000 in the connection string. Apparently taking out the Z fields, running it, then putting them back in made the custom connect set the packet size correctly.