I'm having the same types of issues with two LS410D units on the same network.
One is currently only a DLNA server.
The other runs BitTorrent and serves as the backup to my main backup unit (an older Buffalo drive) and as a backup for the DLNA server. I also use it for video file conversion - the file to be converted and the converted file are stored there while another machine on the network does the conversion.
The DLNA server will randomly quit serving. Sometimes I can reconnect to it immediately (using Roku), sometimes it takes a few minutes for it to come back up. The Roku and the rest of the network is hardwired - NOT using wireless. Sometimes the Roku reports file corruption as the error, but when it comes back up the file plays fine. The units are all set to static IPs (MAC binding on the Roku).
(I have eliminated the Roku as the issue, I have 2 different Roku versions, both on static IPs and neither using wireless, and DLNA failure happens with both and in the exact same way.)
The BT/backup unit does the same thing - randomly goes offline. It sometimes happens during backups. It sometimes happens when files are being accessed or moved. It sometimes happens when a video is being converted. There is no pattern. Sometimes I can convert videos for 12 hours straight before it quits, sometimes 10 minutes. Sometimes there's nothing going on at all and I try to connect and it just isn't there. Keep in mind I don't have something like video conversion going all the time on this unit, just sometimes, and even then the box that handles that is running an old 100 mb (not gigabit) network card so it isn't like there's a lot of traffic going to/from it in any case. It should be perfectly capable of handling 1000% of the traffic I throw at it, and there are no other users on the network.
Both units are showing to be using firmware 1.34-0.95, although oddly the "check for update" doesn't seem to work because one was using outdated firmware for a long time and always said there was no update. Finally it said there was an update when I logged in, but I had been prompted to install the update to the other unit a full month prior and had logged in many times on both units. In any case, the firmware updates didn't solve the issue.
To answer your questions, on my units the following settings:
File Sharing: both running SMB, one also running FTP. No special users or groups have been set up.
Services: one unit running DLNA only, the other nothing
Applications: non-DLNA unit running BitTorrent only and only sometimes, other unit nothing
Backup: DLNA unit backs up to the BT unit (daily differential), otherwise nothing
Management: UPS sync off, sleep timers off
Also, frame size on the network lan settings are both set to 1500
These units aren't really doing anything special and aren't used as office file servers. They see very little traffic except for when videos are being converted or watched via DLNA, and even then it's only one user. Nothing special is going on with them, but they constantly and randomly have this same issue.
Since one is running some services/apps and the other is not (or neither is running it), and they're having the same issue, it would seem the following can be eliminated as issues:
1. backup
2. FTP
3. BitTorrent
4. DLNA
5. Users/Groups (not in use)
6. AFP (not in use)
7. Web Access (not in use)
8. USB Drives (not in use)
9. itunes (not in use)
10. Print Server (not in use)
11. DHCP issues on the network (not in use)
12. Time Machine (not in use)
13. Direct Copy (not in use)
14. Everything on the Management tab, I'm not using any of that stuff (sleep timer, ups sync, etc.)
These units have some kind of fundamental hardware or firmware flaw that is causing the issue. I really hope it's a firmware issue, I have 3 Buffalo units and these 2 have been nothing but trouble.