Access denied errors when not using pre-installed backup agent (4240794)

You receive the following errors when running backups, when using the non-persistent backup agents.

Failed to install backup agent. Details: Access is denied. Make sure you have specified the correct access account for Backup Agent.

You have confirmed that the account set in the schedule settings is correct.

This can be caused by an improper schedule configuration, where the task is set to recur past the time when the backup would begin again.
e.g. You set the job to recur indefinitely every 4 hours, but the schedule frequency is set to "daily".
This will cause the schedule to never truly end, and the backup agent to never get undeployed.
When the schedule begins, the agent is deployed, and since the agent is running from a previous backup already, you may receive this error.

Check the schedule settings.
If you do need to run the schedule more than once per day, set the "Repeat task" duration field to end after the appropriate amount of time.
This needs to end before the next scheduled run time.

For instance, in a situation where you want to run the backup every day, every 4 hours, set the trigger settings as follows:
Daily, Every 1 days, starting today
Repeat task every "4 hours" for a duration of "1 Day"

If this issue currently exists, you may need to remove the backup agent service from the DCs manually.
Since the task never completed, the backup agent never got removed properly.
Run the following command on the DCs with a stuck backup agent.

cmd> sc delete ErdAgent