Try manually downloading the firmware and manually updating it through the router GUI.
I ran into the same problem as the OP with my home RT-AC68U and one at work. Actually, the router just goes down at midnight for 5 to 15 minutes, once or twice before self-recovering and not updating.
My home router and work router has stuff like this in the logs (some info redacted):
"Apr 21 00:00:07 WATCHDOG: [FAUPGRADE][auto_firmware_check
7790)]periodic_check AM 2:4
Apr 21 00:00:07 WATCHDOG: [FAUPGRADE][auto_firmware_check
7834)]do webs_update
...
Apr 21 00:00:07 WATCHDOG: [FAUPGRADE][auto_firmware_check
7790)]periodic_check AM 2:4
Apr 21 00:00:07 WATCHDOG: [FAUPGRADE][auto_firmware_check
7834)]do webs_update
Apr 21 00:00:15 kernel: DROP IN=eth0
...
Apr 21 00:00:24 WATCHDOG: [FAUPGRADE][auto_firmware_check
7852)]retrieve firmware information
Apr 21 00:00:24 WATCHDOG: [FAUPGRADE][auto_firmware_check
7867)]fimrware update check first time
Apr 21 00:00:24 rc_service: watchdog 243:notify_rc stop_upgrade;start_webs_upgrade
Apr 21 00:00:25 BWDPI: [BWANA] Revert process (pid=11184)
Apr 21 00:00:25 BWDPI: [sql_remove_journal(63)] remove /jffs/.sys/TrafficAnalyzer/TrafficAnalyzer.db-journal
Apr 21 00:00:25 BWDPI: [BWANA] remove /jffs/.sys/TrafficAnalyzer/TrafficAnalyzer.db
Apr 21 00:00:26 usb: USB is ejected
Apr 21 00:00:26 kernel: DROP IN=eth0 ...
...
Apr 21 00:00:27 WEBDAV Server: daemon is stoped
Apr 21 00:00:27 WEBDAV Server: arpping daemon is stoped
Apr 21 00:00:27 iTunes: daemon is stoped
Apr 21 00:00:27 FTP Server: daemon is stopped
Apr 21 00:00:27 Samba Server: smb daemon is stoped
Apr 21 00:00:27 kernel: gro disabled
Apr 21 00:00:27 Timemachine: daemon is stoped
Apr 21 00:00:27 WEBDAV Server: daemon is stoped
Apr 21 00:00:28 iTunes: daemon is stoped
Apr 21 00:00:28 FTP Server: daemon is stopped
Apr 21 00:00:28 Samba Server: smb daemon is stoped
Apr 21 00:00:28 kernel: gro disabled
Apr 21 00:00:28 Timemachine: daemon is stoped
Apr 21 00:00:32 ahs: ===Terminate ahs daemon===
Apr 21 00:00:32 disk_monitor: Finish
Apr 21 00:00:32 Mastiff: Got SIGTERM
Apr 21 00:00:32 Mastiff: Got SIGTERM
Apr 21 00:00:32 syslog: Got SIGTERM
Apr 21 00:00:33 WATCHDOG: [FAUPGRADE][auto_firmware_check
7898)]no need to upgrade firmware
Apr 21 00:00:33 rc_service: watchdog 243:notify_rc stop_aae
Apr 21 00:00:33 rc_service: waitting "stop_upgrade;start_webs_upgrade" via watchdog ...
Apr 21 00:00:33 hour monitor: daemon is starting
Apr 21 00:00:34 AUTO_UPGRADE: To download fw/rsa, Start
Apr 21 00:00:34 AUTO_UPGRADE: cfg_trigger=
Apr 21 00:00:34 AUTO_UPGRADE: ! IS_BCMHND
Apr 21 00:00:34 rc_service: watchdog 243:notify_rc start_mastiff
Apr 21 00:00:34 rc_service: waitting "stop_aae" via watchdog ...
Apr 21 00:00:35 AUTO_UPGRADE: wget fw Real RT-AC68U_3004_386_51685-gd1be76f_un.zip
Apr 21 00:00:38 Mastiff: init
...
Apr 21 00:00:50 AUTO_UPGRADE: exit code: 0
Apr 21 00:00:50 AUTO_UPGRADE: wget fw Real RT-AC68U_3004_386_51685-gd1be76f_rsa.zip
Apr 21 00:00:50 AUTO_UPGRADE: exit code: 0
Apr 21 00:00:50 AUTO_UPGRADE: mv trx OK..."
I even found this article last night:
https://www.asus.com/us/support/faq/1045585/.
Today, I finally downloaded
"ASUS RT-AC68U Firmware version 3.0.0.4.386_51685
Version 3.0.0.4.386_51685
98.85 MB
2024/04/15" from
https://www.asus.com/supportonly/rt-ac68u/helpdesk_bios/ and uploaded it back to the router via Asus's web UI and it worked. I'm now on 3.0.0.4.386_51685.