@Kevin_Z 

hi kevin,

here are your asked information:
sandisk extreme, formated with exfat. pc is hooked up to lan with patch-cable.

BUT this all does not matter, i was able to solve it:

i have changed the security policy via gpedit.msc for lan mangager authentification to ONLY reply to NTLMv2

now, it is working fine on be800 too like on my old fritzbox7490. you probably should apply a compability mode the accepts requests/reponses for non NTLMv2 security via optional checkbox.
my windows 10 pc will NOW be able to access the storage on BE800 my pico 4 with moon vr player will not...

general critism, not completly related to the found resolution:

compability out of the box is important! an in case of smb sharing, there should be more transparency.

for me, coming from an asus gt-axe11000 (that is advertised as a "gaming router"), it is bit disapointing how much less you can configure in general and how intransparent tp-link gui is. the gui does not really do a good job in informing the user about the requirements, it is too intransparent, imho. for example other wifi routers show, which smb-protocol is supported, down to the insecure smb-v1, with an extra check-box, the tp-link gui shows nothing (and how hard is it, to add an html label for a bit additonal information?)! the log was pretty useless, it didn't even show that there was an unsuccessful attempt to access the usb-storage ...

  @Kevin_Z 

After I update to the version 1.1.99 Build 20241126 rel.18304(4341), there is a chance that the Web UI is not fully loaded. 

The Network Map does not show any clients and mesh devices. Both Mesh and Clients sections are empty. Device priority and device isolation list are empty too. I have tried to re-sign in, and wait for 10 minutes to refresh the page. The internet connections work as normal to all my deivices. No connection issues.

Tether app will show clients, easymesh devices,and device priority. However, device isolation list is empty in Tether app.

The only way I can fix this is rebooting the router. After rebooting, the router will load everything properly. I have tried to reboot 4 time. The issue occurs 2 times to me.

QoS section (Device Priority) is also empty too. I used to add some devices in the priority list.

Most importantly, Device isolation is not loaded properly. The Isolated devices list is empty.

  @JFDSD 

I had similar problems until i have cleared and cleaned the browsers cache.

Even after clearing browser cache there is like between 1 to 3 seconds delay till details are displayed in the UI.

It does not bother me and i believe the next build will bring UI optimizations.

  @Kevin_Z 

There are one main router (BE800) and 3 easymesh satellites in my network via EasyMesh Ethernet Backhaul. The other two are old wifi 6 routers (Arcadyan Boost 2.0). The olde routers work superisely well with BE800. No any disconnection. I will replace them with wifi 6e or 7 extenders later if both BE800 and BE550 settle down well n my network.

No I am not usring or share any USB drive or HDD.

  @Kevin_Z 

I disconnected two old routers. I restarted 4 times. The Web UI was fully loaded every time. It looks the multiple EasyMesh routers cause the issue. 

I also found that once I conneected either of the old routers back into the mesh network, all my Iot devices would incorrectly turn into wired clients under the older router in both WEB UI and Tether. In reality, only the main router BE800 boradcasts the IOT ssid, and all the iots are wirelessly connected to the IOT ssid only. Both old routers are set as agent mode in the settings.

  @Kevin_Z 

When I re-connected the old routers back into the mesh network, both Tplink Web UI and Tether (logged into BE800) would incorrectly show my iot devices are Ethernet wired to my old router. All my iots were actually wirelessly connected to BE800 only.