There will be a timeout every day at around 10pm.
Two possible causes, either your server’s resources are badly optimized network-wise and cause an overflow with enough players, or your actual box bandwidth can’t handle the traffic. Monitor both and make adjustments accordingly.
Is there a way to investigate the cause of this?
For the game side, the Beta client (change branch in the FiveM main screen) has some network-related tools you can use to see any discrepancies; the server box itself you’ll need to find software to monitor the traffic.