Just got it.
For some reason, CrystalImageHandler was not defined in the web.config file.
HttpHandler, . ( Version PublicKeyToken Crystal Reports)
<add verb="GET" path="CrystalImageHandler.aspx" type="CrystalDecisions.Web.CrystalImageHandler, CrystalDecisions.Web, Version=11.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"/>