While I appreciate you are getting these warnings at the same time of your upgrade, these emails do not tell us anything by themselves. We would need to know what the process was running which encountered this error to determine the exact details of the issue.
The fact you mention that no tasks run at that time in the software is also pointing to something else so we either need more, detailed information and also rule out the something else.
Additionally, simply because nothing has changed on the server, does not mean that anything is wrong elsewhere. The set it and forget it mentality does not apply to servers 🙂 .