This is an issue about exporting / importing scripts that are not in the default package.
Can't be imported from a workspace from a previous version (didn't work to import in Studio 7.8.3 from a 7.8.0 workspace);
Can't be exported as a bos archive (see "steps to reproduce").
The class works fine in groovy expressions within the process.
Windows 7
Leave the package name empty at the Groovy class creation. Despite the warning "The use of the default package is discouraged", it works well in groovy expression (even better : static methods appear in user defined functions), and no more problem about exporting these classes. Although it seems to be a bad practice.