K2 26023 Process instance n not found for domain\user at nnn.nnn.nnn.nnn

If you see a lot of 26023 Process instance not found errors in your K2 server log, you are probably doing someting illegal.
For example, you are trying to use Connection.OpenProcessInstance() on a Completed Process Instance. A big part of K2 is auditing and thus, it won't allow you to modify completed instances - that is why you have K2 right?
Even if you are using Connection.OpenProcessInstance() for Active Process Instances, you should question if you are using the right methodology. Process Instances should for the most part only be modified through the OpenWorklistItem() or OpenServerItem() mechanism.