Symptoms

Suppose you upgraded your K2 environment and some custom solution which worked well prior to upgrade or even works in another already upgraded environment with the following error returned from SamrtObject:
"Method could not be resolved! - Error accessing proccess datafields via SmartObject server
 

Diagnoses

Whereas causes of such error messages may differ one of the first things to check whether all of your K2 components on all your servers were upgraded to the same version as mismatch of binaries on different servers very likely will lead to errors of ""Method could not be resolved!" type.
 

Resolution

Review installed K2 components in the environment and make sure that all servers were upgraded to the same version of K2 and all required components were upgraded.
For example such issue can be caused by the fact that not all K2 components were upgraded up to correct version at the SharePoint server you may need to remove unneeded components and update required once, plus perfrorm reconfiguration or repair of K2 blackpearl afterwards.