Symptoms
Input string was not in a correct format Error After Installing K2 Update
Diagnoses
We were able to reproduce the issue by using the language for executionCulture (for both runtime and designer sites) which is different from the system language.
Here is a kb article on how this can be changed: http://help.k2.com/kb001458.aspx
Resolution
Changing the SmartObject executionCulture to "en-US" resolved the issue.