Aurora MySQL 5.7 randomly fails

This is the 5th time. It happens once a week (Tuesday or Wednesday within 03:00-07:00 UTC+0). On the console, it shows available but inaccessible. We try to wait if the instance will recover itself, after ~30 min nothing happens. So I reboot it manually, then it came online again after rebooting (~5 min). It would be helpful to know what actually went wrong. This is only a dev server with few users and records. Engine: Aurora MySQL 5.7.12 DB instance class: db.t2.small Backup time: 16:00-16:30 UTC+0 Maintenance time: sun:17:00-sun:17:30 UTC+0 Below is the only list of available logs after rebooting the instance. error/mysql-error-running.log.2018-07-24.03 Tue Jul 24 11:14:06 GMT+800 2018 11.8 kB error/mysql-error-running.log.2018-07-24.04 Tue Jul 24 11:30:00 GMT+800 2018 285.5 kB error/mysql-error-running.log.2018-07-24.05 Tue Jul 24 12:30:00 GMT+800 2018 31.1 kB error/mysql-error-running.log.2018-07-24.06 Tue Jul 24 13:30:00 GMT+800 2018 31.8 kB error/mysql-error-running.log.2018-07-24.07 Tue Jul 24 14:30:00 GMT+800 2018 32.9 kB error/mysql-error-running.log.2018-07-24.08 Tue Jul 24 15:30:00 GMT+800 2018 29 kB error/mysql-error-running.log.2018-07-24.09 Tue Jul 24 16:30:00 GMT+800 2018 32.1 kB error/mysql-error-running.log.2018-07-24.10 Tue Jul 24 17:30:00 GMT+800 2018 27.5 kB error/mysql-error-running.log.2018-07-24.11 Tue Jul 24 18:30:00 GMT+800 2018 31.7 kB error/mysql-error-running.log.2018-07-24.12 Tue Jul 24 19:30:00 GMT+800 2018 27.1 kB error/mysql-error-running.log.2018-07-24.13 Tue Jul 24 20:30:00 GMT+800 2018 22.4 kB error/mysql-error-running.log.2018-07-24.14 Tue Jul 24 21:30:00 GMT+800 2018 22.8 kB error/mysql-error-running.log.2018-07-24.15 Tue Jul 24 22:30:00 GMT+800 2018 24.7 kB error/mysql-error-running.log.2018-07-24.16 Tue Jul 24 23:30:00 GMT+800 2018 24.7 kB error/mysql-error.log Wed Jul 25 00:34:45 GMT+800 2018 2.6 kB external/mysql-external.log Wed Jul 25 00:30:00 GMT+800 2018 7.6 kB *external/mysql-external.log* /rdsdbbin/oscar/bin/mysqld, Version: 5.7.12 (MySQL Community Server (GPL)). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time,ServerHost,User,UserHost,Command,Payload /rdsdbbin/oscar/bin/mysqld, Version: 5.7.12 (MySQL Community Server (GPL)). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time,ServerHost,User,UserHost,Command,Payload /rdsdbbin/oscar/bin/mysqld, Version: 5.7.12 (MySQL Community Server (GPL)). started with: Tcp port: 3306 Unix socket: /tmp/mysql.sock Time,ServerHost,User,UserHost,Command,Payload ----------------------- END OF LOG ---------------------- *error/mysql-error-running.log.2018-07-24.03* shows: https://pastebin.com/ywmXLR5g . *error/mysql-error-running.log.2018-07-24.04* shows: https://pastebin.com/g1dkR6rj . *error/mysql-error-running.log.2018-07-24.18* shows: https://pastebin.com/g0aAXfaT . All other logs shows nothing(see photo). **Event Logs** July 24, 2018 at 11:14:14 AM UTC+8 DB instance restarted July 24, 2018 at 11:13:31 AM UTC+8 Error restarting mysql: Engine bootstrap failed with no mysqld process running... July 24, 2018 at 11:12:01 AM UTC+8 Recovery of the DB instance is complete. July 24, 2018 at 11:04:26 AM UTC+8 Recovery of the DB instance has started. Recovery time will vary with the amount of data to be recovered. CPU Utilization (07-24-2018) CPU Utilization (07-11-2018 to 07-24-2018)