I'm working on a distributed SharePoint environment with a single K2 server. When trying to open a SmartObject (not even execute), I get the variations of the error message "Value cannot be null. Parameter name: s".
I get this error in Smart Forms designer, client side, application event log and when using the Smart Object Tester.
I'd like to add that the SharePoint farm is running in 2013 with K2 in SPS2010 mode. The K2 components (described above) are interacting with a site collection created in SPS 2010 mode.