These sounds very much like somthing we've seen, and is a windows security 'feature'
You can work around this by the simple regedit from the server as documented in this URL (BryansGeekSpeak: Debugging Windows Server 2008R2 Event ID 6037 - "The target name used is not valid"), and given below for reference.
- Go to REGEDIT > HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
- Right click MSV1_0 > New > Multi-String Value
- Type 'BackConnectionHostNames'
- Right click & select ‘Modify’
- Enter the Hostname of the site: WEBSITENAME (and on a new line enter the FQDN, WEBSITENAME.domain.com as well)
- Restart the QlikSense Proxy