I have a document with many hyperlinks to other docs on the same computer.
At one point this document for some unknown reason converted all of the
hyperlinks to show field codes
Therafter the hyperlinks could not be returned to a normal state, regardless
of the "toggle field codes" selection.
The links still work but have rendered the document a mess to look at.

The only way I have found to clear this condition is to do the following for
each link;
1. right-click and select "Hyperlink/edit hyperlink", copy (cont C) the
file or webpage name targeted by the link and
then click the button to remove the link
2. In the document, select the text that used to be the Hyperlink and
right-click
and select Hyperlink
3. paste (cont V) the file or webpage name previously coppied and click
"OK"

This bug is manifest in the following way;
The normal field codes are enclosed in a set of curly brackets
The discrepant field codes are enclosed by the default non-printable
character symbol which is a square followed by the blue display text also
bracketed with squares.
For example;
 HYPERLINK "Test.doc"  Test document