Running a new CEM Recording Session fails with: "Error: Unable to start recording for all enabled monitors"
The normal root cause of this error is at least one TIM is not contactable by the Enterprise Manager (EM) that is running the TIM Collection Service e.g. the TIM is not started or there is some other problem impacting the EM <-> TIM communication.
APM 9.x, 10.x
Check the CEM UI Setup > Monitors page to confirm the status of the TIMs.
One TIM was showing "Domain Configuration Status" Communication Error 404.
A resync of the TIMs was enough to clear the Communication Error and that also allowed the recording session to run.