This is an old one, but some co-workers ran into this again recently on a project.
When you have a SharePoint workflow that assigns tasks (either Designer or Visual Studio workflow), when you change the Assigned To column on the task list to allow Multiple Selections, your workflow will hang and become unresponsive when a user edits the task. You’ll receive a message about the task being locked (“This task is currently locked by a running workflow and cannot be edited”). OnTaskChanged events will no longer fire and your workflow will be stuck and unrecoverable.
There is no hotfix or workaround for this, so you simply cannot have the assigned to column support multiple users or groups. Just use a single group instead.
Following are some other resources discussing this: