It looks like Simple Object Pool includes PlayMaker.dll. This older PlayMaker.dll in that package overwrote the one in your project and caused the errors. I contacted the author to ask them to remove it.
In the meantime, you can revert to the original project, and this time when importing Simple Object Pool, uncheck PlayMaker.dll in the import dialog.
Or you could try re-installing PlayMaker in the project. That should also fix it.