[Hummingbird Pro] Clear transients gives "SyntaxError: Unexpected end of JSON input"

Multisite installation with Woocommerce. When trying to clear transients from subsites – expired or all – gives the error:
SyntaxError: Unexpected end of JSON input

If i do “delete all” option that seems to work better – but doesn’t clear everything the first go-round. For obvious reasons, I’d like the option to leave post revisions and some other tables alone, but still remove all expired transients.