In this case the error would occur if you try to view, send, or save the fax.
Try selecting a different style and view the fax (View Fax button).
For example, select one of the default styles such as
The retry cover sheet is rarely used.
If you can view the fax using a different style then there may be a problem with the cover sheet file of the style you were using. Check the asciitiff - Convert an ASCII file into a TIFF image commands in the cover sheet file.
If there is a problem with a default cover sheet you can get a new version of all the default cover sheets and default cover sheet TIFF images by executing the newcovers script. This script may also be used to test and preview cover sheets.
For more information on cover sheets see The Cover Sheet.