RAX45 DLNA server stops working intermittently

Firmware upgrades should retain and update settings as needed, though in some rare edge cases, corruption can happen, and in those cases, clearing the NVRAM is needed (basically what the factory reset does).

If the issue happens after a factory reset, another thing to test, is connecting a different drive, even if it is just a flash drive, and put a few random media files on it, and see if the issue still happens, it ie doesn't, then odds are likely that you may have a media file that has some corrupt metadata that is causing the DLNA service to freak out. In which case, the only way to find out which file is doing it, will be to head to the debug page and then start capturing debug logs, then saving the zip file, then unzipping that file to a new folder, and then reviewing the logs for DLNA related errors and see which files it failed on.

Just remember, do not share debug logs on a public forum, since they contain all router config data in plain text, including the router's admin password, WiFi passwords, and other passwords for things such as DDNS services.

PS, if you ever need to do a factory reset to solve an issue, do not immediately restore a backup config file since the config files are essentially an encrypted image of the NVRAM, thus if there is corruption there, it will essentially restore that corruption.

Though one option that is useful, is to do abackup of the config file, especially if you have a large number os port forwards, address reservations, and security policies which can take a long time to re-enter. In those cases if a factory reset doesn't solve the issue, you can simply restore the backup.