If you are using the new DB Connection (in Knime 4.x) for multiple DB Queries, you will need to check the Restore Database Connection setting the Advanced options for the DB Connection Node:
If you don’t have that checked, you will run into the following errors: DB Connection no longer available. Go to advanced settings to enable connection restoration
Could you please elaborate how you use the DB Connector node as I can’t reproduce your issue? Which type of database are you using? How many queries are connected to the connector node?