delete scheduled tasks fails when task identifier is account:uid
If you go to /admin/config/workflow/rules/schedule you will see a table of your scheduled components. One of the columns in that table is the task identifier - that's the string you will have to use to delete the scheduled task. You didn't post any details related to the scheduling, so I don't know what you used as a task identifier. The task identifier is necessary because one component may be queued to execute many many times, each time with a different identifier. The component name is not unique. The task identifier is.