I have the same problem (ever since WebDAV was introduced) on a Windows 10 machine. The drive does not “disappear” as much as the name disappears - the drive is visible, but only as “Local Disk” with the original drive letter. Cannot access it though. If an explorer window is open with the files visible I can still see them, but they aren’t “there” and refreshing the window empties it.
Interestingly, this happens only on my laptop which is on WiFi and only when it idles. Never happens when I am continually working on it. If I leave my laptop for a while and come back, and the laptop has locked screen, I can guarantee that the drive is lost. This has nothing to do with the WiFi per se (I turned off all the radios and everything worked fine). My desktop does not have this problem - it too idles, but is on a hardwired connection.
Hope this provides some additional information on troubleshooting. Thanks, by the way for an excellent piece of software!