Looks like Aspose has changed their Maven repository URL. Documentation still points to the old one. Maybe this was announced and I just missed it.
New Maven URL appears to be: http://artifact.aspose.com/repo/
The old one may cause Maven to cache bad HTML files as JAR files (it did for me, at least). If this happened to you, you may need to clear out $M2_HOME/repository/com/aspose
Thought I’d post here in case it helps anybody else.