Do you have selected an Auto-Detect setting (View > Character Encoding > Auto-Detect) ?

I have exactly the same settings on the desktop iMac as on the Macbook pro, but the problem only occurs on the macbook.

Which encoding is selected when you have that problem and which encoding do you need to select instead?

It is not clear to me which encoding are selected - there is no conspicuous check-mark or any other indication.