Please open the following file :
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\ISAPI\Virto.Calendar\web.config (for SharePoint 2007)
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\ISAPI\Virto.Calendar\web.config (for SharePoint 2010)

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Virto.Calendar\web.config (for SharePoint 2013)


in notepad.
Add the following element to "system.web" section:
<customErrors mode="Off" />

This element allows to get extended error info. Please send us extended info.