Hi Stevek, how are you trying to open the report? Via Report Manager or by code?
In my personal experience, I once got that error, and the reason was that I didn't have the pluginid of the report property filled in on the database field. I explain myself, I'd written a piece of code that opened the report invoking Crystal Reports Previewer, and, among many other parameters, I needed to pass the pluginid which was the report. Well.. I wasn't sending this parameter correctly. Once I did it, it started working.
Please, ensure that Crystal Reports Previewer is getting the report (plugin id) correctly. Note that in GetReport.ascx as in Reporting.ascx there are some hidden fields, which are these parameters I tell you..
<input type="hidden" name="usercode" id="usercode"/>
<input type="hidden" name="pass" id="pass"/>
<input type="hidden" name="pluginid" id="pluginid"/>
<input type="hidden" name="keyfield" id="keyfield"/>
<input type="hidden" name="rsf" id="rsf"/>
<input type="hidden" name="wsql" id="wsql"/>
<input type="hidden" name="sqlqry" id="sqlqry"/>
<input type="hidden" name="forcesql" id="forcesql" value="0" />
<input type="hidden" name="sortfields" id="sortfields"/>
<input type="hidden" name="sortdirections" id="sortdirections"/>
<input type="hidden" name="ss" id="ss"/>
<input type="hidden" name="reportpath" id="reportpath"/>
A good idea would be making them visible so you know what is going on..
Hope it helps finding the solution.
c-ya!