Update won't install, no error given - are there any log files?

Trying to update by TP-Link Archver C7 v5 from 19.07.7 to 19.07.8 (also tried to 21.02.0). Update gets uploaded successfully and starts, however after some time I am returned to the login page and it still shows the old version (19.07.7). How can I analyze this issue? Are there any update logs?

How are you updating it?

From the Luci webinterface using System -> Backup/Flash firmware -> Flash new firmware image with the retain settings option enabled.
Update: Also tried via SSH, but as the SSH connection gets disconnected quite early, so this doesn't help. However after trying it two times, it suddenly updated. Strange.
Anyway I am still interested in this for the next time: Where can I store the output (using redirection) from the sysupgrade command? Tried to store it in root, but it was gone after reboot.