Dear friend!
Since I don't have any more information which you provide, I can only recommend that you focus on troubleshooting in the following directions:
1. Check whether the license resource (LNSDFDMSF01) is used up. If yes, release the license resources in other monitoring directions. Alternatively, purchase a new license.
2. Check whether the board (not as good as the ST2) that is responsible for the check function works normally. If the board is abnormal, rectify the fault.
3. The network planning is improper, or the physical fiber and logical fiber are Incorrect or insufficient connections exist. Please check.
Thank you!