I'm currently running YouTrack 3.0.2 (build #1651 [13-Jul-2011 20:22] )

What version of the server are you running against? I'll try and reproduce this.

I see the same problem when creating a new issue and then attaching a file to it.

If I fail to re-authenticate before calling "AttachFileToIssue" then I get an authorisation failure.

' Connect to YouTrack server
Dim YT_Connection As New Connection("server", 8080)
YT_Connection.Authenticate("user", "pass")

' Create the issue
Dim NewIssue As New Issue() With {
   .Description = "This is a test issue designed within a prototype to test auto submission of tickets.",
   .Summary = "A test issue",
   .ProjectShortName = "ENV",
   .Type = "Bug"
}

' Create an instance of IssueManagement and send it the issue
Dim YT_IssueManager As New IssueManagement(YT_Connection)
Dim YT_TicketID As String = YT_IssueManager.CreateIssue(NewIssue)

' getScreenshot() takes a screenshot and returns a filename with full path
Dim ScreenshotFilename As String = getScreenshot()

' If I remove this call to Authenticate then the AttachFileToIssue will fail with "Unauthorized Unauthorized" error from EasyHTTP
YT_Connection.Authenticate("user", "pass")

' Attach file to new ticket
YT_IssueManager.AttachFileToIssue(YT_TicketID, ScreenshotFilename)