This can happen when the temp directory is not writable, doesn't exist, or the file system has no disk space available.  To resolve, look at the 'Temporary Directory:' line on the About page (the link is in the upper right corner of Unanet's user interface).  Make sure the temp directory exists, is writable by users (vs admins), and that the disk or file system where it is located has sufficient disk space to create the temporary files that Unanet charts require.

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