Q: Lenovo Yoga 3 / Win 8.1 Pro

Hello there, axelweil.

It sounds like when you connect your iPhone to your computer running Windows 8.1, iTunes becomes unresponsive. The following Knowledge Base article offers up some great troubleshooting recommendations:

iTunes: May become unresponsive when connecting iPhone, iPad, or iPod touch

Follow the steps below to troubleshoot the issue:

1. Install any available software updates for your computer. 
    
   • OS X: Use Software Update.
    
   • For Windows: Refer to the documentation for your computer for instructions on using Windows Update to update your computer.
2. Update iTunes to the latest version. 
    
   • OS X: Use Software Update or download iTunes from the Apple website.
    
   • For Windows: Use Apple Software Update for Windows or download iTunes from the Apple website.
3. Test with automatic syncing disabled. 
    
   • For OS X: Click the iTunes menu and choose Preferences. Click the Devices tab and place a check next to option to "Prevent iPods, iPhones and iPads from syncing automatically." Click OK.
    
   • For Windows: Click the Edit menu and choose Preferences. Click the Devices tab and place a check next to option to "Prevent iPods, iPhones and iPads from syncing automatically." Click OK.
   Reconnect your device and test to see if the issue still occurs. If your device is recognized successfully, you may be able to isolate which type of data is causing the issue. Do this by deselecting all syncing options under each of the syncing tabs and then sync. After that, re-enable your sync options one at a time until the issue returns.
   
   If the issue only occurs when syncing data from the Info tab is enabled (For example: Contact, Calendar, Mail account or Bookmark syncing), reset Sync History on OS X or  on Windows.
    
4. Check your device for podcasts. If there are any podcasts, manually delete all podcast episodes from the device by sliding your finger across the podcast episode's name and tapping Delete. Once all podcasts are removed from your device, reconnect it to the computer, open iTunes, and test.
5. If possible, test to see if connecting your device to a different computer with the latest version of iTunes installed resolves the issue.
6. Test with iTunes plug-ins removed. 
    
   • Quit iTunes.
    
   • Navigate to the following folder, based on your operating system:
       Windows XP:
       C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iTunes Plug-ins
       Windows Vista or Windows 7:
       C:\Users\[username]\App Data\Roaming\Apple Computer\iTunes\iTunes Plug-ins
       Mac OS X: 
       
     • ~/Library/iTunes/iTunes Plug-ins (The "~" represents your Home folder)
       
     • /Library/iTunes/iTunes Plug-ins
      
      
    
   • If any files are in this folder, move them to the Desktop temporarily.
    
   • Open iTunes and test to see if the issue is resolved. If so, you may leave these iTunes plug-ins removed. If you wish to re-use these iTunes plug-ins, consult the vendor for the iTunes plug-ins to see if any updates are available.

Thanks for being a part of Apple Support Communities.

Kind Regards.