||Don’t worry in case you are receiving temporary error 18 on your PC screen.
Usually, the error automatically gets corrected itself but you can use the simple steps to troubleshoot an issue.
Steps to fix temporary error 18:
• On your desktop and mobile devices, you need to sign out of Yahoo mail.
• Just check that you are using a supported OS and Web browser.
• Now you need to clear Web browser's cache.
• Afterwards, restart your Web browser.
• Finally, sign back into Yahoo mail for seeing in case an error is gone.
Read more here :- http://yahooemailhelp.microblr.com/post/86090/5_Ways_To_Fix_Unresponsive_Yahoo_Mail_App_On_Iphone