Your public electricity network (voltage, frequency) has exceeded the specifications imposed by the standard selected in the inverter. This causes the inverter to become out of synchronization with the grid and to go into a safe state. Once the grid is stable, the inverter will resynchronize.
If you often see these errors, check that the standard set in OS ONE is in accordance with the one requested by your public grid operator. If this is the case, you should contact your electricity provider to have the problem resolved.
If the regulations allow you to do so, you can change the inverter's desynchronization ranges (voltage & frequency) in advanced mode or simply choose the IM18UG-1 standard in OS ONE (See how to change the standard).