Nimbostratus
Cirrostratus
Without any detail, I'd guess it's an issue with the monitor send string behavior change:
sol10655: Change in Behavior: CR/LF characters appended to the HTTP monitor Send string
https://support.f5.com/kb/en-us/solutions/public/10000/600/sol10655.html
Beginning in BIG-IP version 10.2.0, the BIG-IP system no longer automatically appends any CR/LF character set to the HTTP monitor Send string. This was an intentional change to accommodate custom monitors such as those that use the Send string to send a POST request with a message body.
If you're performing a major upgrade, I suggest testing it in a lab environment if you have the same or similar platform available. Also, if you run into issues, make sure to run qkview before and after the upgrade attempt.
And try to upgrade the standby unit in an HA pair first. This allows you to troubleshoot any issues while the active unit handles the production traffic. Once you've confirmed the upgrade succeeded, you can fail over the active unit and upgrade the peer.
You can check the manuals on AskF5 for more detailed instructions and open a case with F5 Support to get additional help.
Aaron