Hello @dn1
Typically, the "Method could not be resolved!" error is due to a version mismatch between servers. Double check via Control Panel -> Programs and Features on all K2 related servers that the versions match.
Please ensure that all K2 products have the same version, this includes (K2 BlackPearl, K2 SmartForms, K2 SmartForms Control Pack, K2 for SharePoint), should there be a one that is not the same version then you will need to update them as well.
Also have at this solved post: Smartobject throws "Method could not be resolved!" exception:
https://community.k2.com/t5/K2-Blackpearl-Forum-Posts/Smartobject-throws-quot-Method-could-not-be-resolved-quot/td-p/73476?nobounce=
All the best of luck.