IIS7, WebDav via httpS, Basic Authorization, self-writen .net C# as WebDav client.

I'm make propfind request to folder hxxs://server.net/files/Home/My and get successful response with status code 207. All Ok.
After it don't close application and wait more what 60 min.
After 60 min inactivity make same response (byte to byte) to server ad get 500 Internal Server Error as response (in server logs 500.5).
After recieve this error 500 any propfind request to server -
hxxs://server.net/files/Home/, hxxs://server.net/files/Home/aaa, hxxs://server.net/files/Home/My/bbb etc. return same 500 error (expect root of the server).

If i'm requesting root of server - hxxs://server.net/ - i'm getting correct response and all other request like as hxxs://server.net/files/Home/My start to working. But not always, pereodically request of the server root not resset 500 error.

Looked as session timeout on server side. But i'm don't use any session supporting on client side (cookies not accepted and not sended, not used special headers where session info can be stored).

Any suggestion reason - why it happened?