An inaccurate computer system clock can cause activation problems. Did you reset your computer’s date and time recently? Check that your computer's system clock displays the correct date and time, and is set to the correct time-zone.
The message indicates that your Creative Cloud app does not launch because there appears to be a mismatch between the clock on your computer and the Adobe servers.
Time-related errors, for example error 251, may also be displayed if you roll back the clock in an attempt to extend an app trial.