$ ~/bin/archgenxml-virtualenv/bin/archgenxml uml/OPSB.uml
xml.parsers.expat.ExpatError: not well-formed (invalid token): line 51, column 0
For a more complete stacktrace see http://paste.plone.org/26649.
Turns out it's a bug in ArgoUML 0.28 BETA 2: When saving as a .uml file, a sequence of "^@"'s is inserted. Remove it manually, or use 0.26 for now.
See http://argouml.tigris.org/issues/show_bug.cgi?id=5710 for the bug report.
The folks at Tigris were quick to follow this up. They've targeted it for the next beta release (0.28 beta 3).
Beta 3 is out, this bug is fixed.