To fix the problem, please do the following:
First, uninstall Maple 6.0. Then, install Maple 5.3 and open your old .mpl file in it. Click the root node and select File\Export\Branch. You will get a fixed file that will be converted correctly in Maple 6.0.
Maple Pro 5.3 can be downloaded here: http://mirror1.crystaloffice.com/outdat ... epro53.exe