1. Sometimes the error is because of network issues, some connection problem or no proper inputs. If you just go to the error profile in the workspace and retry the error it fixes the problem.
2. If above approach does not resolve the error you can use the K2 Process Management tool in Visual Studio.
As shown in the figure below. you can select process clicking View->K2 Process Management tool in visual studio. Select the process in error and open the process and fix the error.
After fixing the error, go back to the k2 process management and click redeploy the process.
You can provide the label and deploy the process. It will fix that instance of the process. Then you can fix the processes with similar error hitting the retry button and selecting the fixed version of the process.
How do you debug the process if the Visual studio is not installed on the K2 box?