Not sure about the plugin and the attempt to use an old template.
How big of a file is this that is being downloaded?
Is it possible to put one of the sample files in the download directory?
Have you tried replacing the contents of the .htaccess file ensuring to upload/transfer in ASCII mode?
The other thing to try (seeing that I was able to download a .txt file elsewhere on the site) is to modify the .htaccess file as follows:
Find:
<IfModule mod_headers.c>
<FilesMatch "(?i).*\.(zip|pdf|mp3|swf|wma|wmv|wav|epub|ogg|m4v|m4a)$">
# tell all downloads to automatically be treated as "save as" instead of launching in an application directly
# ALERT: ForceType requires Apache2 or later. If using older version of Apache, it will need mod_mime installed. Or just comment out the ForceType line below
# (to disable, just comment the next 2 lines by adding a '#' at the beginning of each):
ForceType application/octet-stream
Header set Content-Disposition attachment
</FilesMatch>
</IfModule>
Modify by adding a # symbol as suggested in that text:
<IfModule mod_headers.c>
<FilesMatch "(?i).*\.(zip|pdf|mp3|swf|wma|wmv|wav|epub|ogg|m4v|m4a)$">
# tell all downloads to automatically be treated as "save as" instead of launching in an application directly
# ALERT: ForceType requires Apache2 or later. If using older version of Apache, it will need mod_mime installed. Or just comment out the ForceType line below
# (to disable, just comment the next 2 lines by adding a '#' at the beginning of each):
# ForceType application/octet-stream
# Header set Content-Disposition attachment
</FilesMatch>
</IfModule>
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