Getting WFA to just 'shut up and do what it's told' might lead to worse implications than just a frustrating failure in importing a .dar file. WFA doesn't support backward compatibility for importing .dar files i.e. you can't import a .dar file created on a newer WFA version into a WFA of an older version.
In this particular case the user has created a .dar from WFA version 2.1(new) and then trying to import it to WFA2.1RC1 ( older ). This is not supported.
To solve this, the best way is to upgrade old WFA 2.1RC1 to WFA2.1 and then try to import this .dar. The user can take a backup on then 2.1RC1 and then restore it on to the newly installed WFA2.1 . After this all his imports will succeed.
WFA2.1 GA is already available and had some important fixes gone in from 2.1RC1 version. So its definitely better to upgrade.
If this post resolved your issue, help others by selecting ACCEPT AS SOLUTION or adding a KUDO.