This warning indicates an agent side problem accessing the repository.
EC2038 Mount::set couldn't open repository "[repo]": [error]
A specified directory may not be accessible to an agent and cannot be mounted - typically a temporary issue.
This issue is most likely rare, and may be caused by the agent machine being overloaded. So if you start to see this issue, try monitoring for how frequently. If it continues to be a rare occurrence, you may choose to simply ignore the warning - otherwise, you could try reducing the number of agents being used with the machine in question to see if that helps to avoid the event being seen.