After I draw a line with the line tool I doubleclick the line to insert text
and the text goes in but is displayed upside down.
try this:
menu File -> Shapes - Show Document Stencil
This will show you the master for all the shapes you have ever used in
this document.
One (or more) will be the connector type giving you this problem.
Double-click the shape:
This will open the shape edit window.
Click on the shape to select it and from the toolbar select the Text
Block Tool (it's selectable from the Text tool - 'A' symbol - nobody
knows it's there until you point it out to them). Rotate the text
handle by 180 degrees.
Close the shape edit window, it will ask you if you want to update all
instances - select Yes.
This should update all the shapes to use the new angle. But if any of
the shapes have had their text angle set individually then that shape
will be unaffected by the global update.