After a week of pulling my hair out, may be sorted.
More testing required, however:

This works (tested on 1 other puter):

DataEnvironment1.Connections(1).ConnectionString = blahblahblahblah




This givers error 13:
DataEnvironment1.Connection1.ConnectionString = blahblahblahblah



I only have one DataEnvironment named DataEnvironment1

Thanks to all who tried to help.
Will post if confirmed resolved!