Symptoms

During workflows deployment assemblies become unavailable. During long deployments of workflows via K2 Package and Deployment in production environment following errors appear in Workspace:

Could not execute the function '%Function_Name%' because the containing assembly 'SourceCode.Workflow.Functions' could not be found.

Once deployment completed re-run of workflows works without any errors.
 

Diagnoses

Normally in such situations it is necessary to make sure that file SourceCode.Workflow.Functions.dll is present on K2 server/servers in the following default locations:
C:Program Files (x86)K2 blackpearlBinFunctions
and
C:Windowsassembly
If this file exists in location specified above but you still getting these errors reboot your K2 server or servers in case of farm install, this should resolve this issue.
 

Resolution

Make sure that file SourceCode.Workflow.Functions.dll is present on K2 server/servers in the following default locations:
C:Program Files (x86)K2 blackpearlBinFunctions
and
C:Windowsassembly
If this file exists in location specified above but you still getting these errors reboot your K2 server or servers in case of farm install, this should resolve this issue.