Getting “task is currently locked” error for SharePoint Workflow created with SharePoint Designer

I am creating a simple workflow using SharePoint Designer that triggers when a user uploads a new Document to a library. This workflow assigns a task to multiple users and then waits until those tasks are complete. Once they are complete it then assigns another task to different users and then waits until those tasks are complete. It repeats this multiple times.

This works great, the problem is that there is no Due Date on the tasks created and they would like a reminder email. Following the Microsoft Documentation I create a secondary workflow on the Task library and set a Due Date on the tasks based off of the Title and send out the reminder.

When I attempt to complete the assigned tasks, it lets me complete the task for one of the users, but after that I receive a "This task is currently locked by a running workflow and cannot be edited" message.

The sharepoint site is part of the Microsoft hosted services, so I have no controll over patches or hotfixes. I believe that it is only a WSS site, not MOSS.

I am new to SharePoint, so any fixes, workarounds, advice, etc. would be appreciated.