Can someone please translate "specified cast is not valid" into English?

I’m yet another generally happy user who is unable to get the eM Client and Google calendars to sync. Have tried all the suggestions I can find here, checked the account settings, run the Diagnostics tab . . . but still no sync. Only the cryptic “specified cast is not valid” message. I might be able to solve it if I had a clue what “cast” means. (As in movie stars??)
Thanks for any tips!

Here’s the solution, if you’re getting this error message when your calendars won’t sync: upgrade to this version –…
I was using a version that I had downloaded a week ago via Google’s apps site.

(Still don’t know what a “specified cast” is. If not those film stars, perhaps that plaster of Paris thing you put on your arm after whacking your laptop too hard??)