"Unable to validate certificate"

[phs3stuff]

After running flawlessly for a while, I'm suddenly getting:

Unable to validate certificate

The certificate for clients6.google.com could not be validated. The certificate chain presented is invalid.

This happens repeatedly at random intervals. Sometimes I get several of these dialogs at once, sometimes just one.

I know what an invalid certificate chain is, just not what to do about it for this. Seems like the server is presenting the invalid chain, which I can't really fix anyway, eh?

Ideas?

Windows 7, fully patched; Help/About says:
Pidgin 2.12.0 (libpurple 2.12.0)
Unknown

(what's the "Unknown" about??)
And I'm using the Hangouts plugin.