According to the relevant information on the Internet, ITunes prompts "Unknown error 50" (Figure 1), mostly due to the related configuration document parameter errors, and the easiest way to solve this problem is to reset the configuration document. Note that there are different paths for saving the itunes parameter configuration document itunesprefs in Windows XP and Windows 7 or higher systems:
If the XP system, we need to enter the C:\Documents and Settings\ username \applicationdata\apple computer\itunes\, delete the Itunesprefs.xml file can be (Figure 2); If it is Windows 7 or later, you need to enter the C:\Users\ username \appdata\roaming\apple computer\itunes\ Delete itunesprefs.xml file. It is recommended that you reboot the system after deleting the file, and then you will be prompted to reconfigure when you run itunes, and you can use it once you are done.
If you are using the above method, please try to go to "itunes→ edit → preferences → parental control", choose to disable the itunes Store (Figure 3), leave "Allow access to itunes U", and then restart itunes. Repeat the previous action and cancel disabling the itunes Store, which can also resolve the "Unknown error-50" problem.