You see an almost blank page with the following message when using Roundcube:
SERVICE CURRENTLY NOT AVAILABLE! Error No. 
The Error 501 comes from Roundcube and means, that Roundube has a problem loading the template.
To solve this problem you need to clear the browser cache and then try again.
Depending on the Browser you are using there are several and different methods to force the browser to read the real content on the server instead of the cached files.
For Mozilla Firefox you could try one of the following examples:
Hold down the Shift-key and left-click the Reload button
Press Ctrl + F5 or press Ctrl + Shift + R (Windows,Linux)
Press Command + Shift + R (Mac)
Clear the Cache: Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"
Remove Cookies: Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"