I've had that error when there's no value in the connection string

You'll need a default connection even if you are setting the actual connection with an expression that evaluates at runtime.

I have a similar sounding job which is a set of packages that import about four hundred csv files daily from four different business section zip file imports (from our outsourced business system) and the same file name and format is in each section i.e. file salsorders.csv will be the same format in each business section.

I use script tasks at the start to set up the correct import, error log and zip archive folders into variables and then a for each section number loop which does the import to combined staging tables which the next package converts and loads to live.

The connection string of a flat file is then an expression such as import folder variable plus file name plus section number variable plus extension.

I split the import section into sub-packages due to the slowness of working on each package and the whole thing is far more manageable. The dtutil batch script to deploy the packages is my most time saving utility!