If you want an error message in the web interface you can set
debug = yes
in section [web] of your config.ini. This is turned off by default as it
might reveal information in the traceback that a user should not see.

If debug is turned off, the traceback is sent to the admin_email
configured in config.ini. If admin_email doesn't have a domain, the
config-item 'domain' in section [mail] is used. So this tries to send
mail to roundup-admin@rokpa.ch according to the config.ini you attached.

Generally: Please direct questions concerning roundup to the
mailinglist. You can subscribe at
https://lists.sourceforge.net/lists/listinfo/roundup-users
and there is also a link the the mailinglist-archive on that page.