hi Ian, I found the same problem of late in my blog www.MadrasNow.com hosted in Blogger & managed to identify the problem and the fix.
A) The problem is there is no ‘Search Description’ in the individual posts, especially if you use WLW or Open Live Writer to blog. The solution is to open up individual posts in Blogger’s ‘edit’ mode and enter the ‘Search Description’. [BTW, this post ‘search description’ is different from the site ‘Description’ which we enter through Settings > Search preferences > Description]
B) But if you forget to enter individual post ‘search description’ by opening up Blogger interface, this twitter:text:description error shows up. And doing it is a nuisance.
C) A code fix is to use the site ‘Description’ as post ‘Search Description’ if it is missing, however, it might lead to ‘Duplicate Description’ error in Webmaster Tools. So a temporary fix, I’ve put in today is to include the page Title itself as post ‘search description’, if it is missing.
D) Here is the actual code, which I put in through Template > Edit HTML
b:if cond=‘data:blog.metaDescription’>
meta expr:content=‘data:blog.metaDescription’ property=‘twitter:description’/>
b:else/>
meta expr:content=’"Please visit " + data:blog.pageTitle + “to read interesting articles.”’ property=‘twitter:description’/>
/b:if>
Since I cannot post code here the opening angle bracket or lesser than sign has been removed - insert them at appropriate. Above code replaces the original twitter card code which is shown in BOLD.
Hope it helps. Please let me know if you find a better fix.