When a mail with a configured action as a response to a assigned task is sent, an error from the K2 server is returned:
"The full error from the K2 server is '(Unknown)'"
The error generally appears on workflows even if a mail is successfully sent.
Investigating the mail should reveal that there's no serial number. (If users don't customize the notification email [and K2 sends out a default one] then it will be a string in very small font and light grey in colour at the bottom of the email).
It's a serial number that should start with "SN=xx_yy" and is easy to find using the search function on a mail.
If you choose "customize email" it should autopopulate serial numbers. In other words: The serial number can be provided in the task notification email via out of the box notification or when customizing the notification you can use the context browser to place the serial number in the notification.