That's a very generic error. To get a bit more info make the following
changes to your Web Front end server in the Web.config file of one of
the WebApplicaitons generating the error.

Search for CallStack= and change the setting from false to true.

Search for CustomErrors= and change the setting from Off to RemoteOnly.
(This means that the CallStack will only show on a browser on the server
console. Remote users will still get the SharePoint error page.

Then do an IISreset.

Finally, go to the WFE server console and browse to one of the pages
generating the error. Instead of seeing "An error has occurred". You
will see a full stack dump and exception statement for the error that is
occurring. That may give you enough information to find out what is
going on.

I hope you don't mind but I posted your tip on my blog.

blog.gavin-adams.com/.../enabling-web-application-error-re
porting/

Well, I did look for the web.config file and there are 20 plus files
with that name. How do I determine which file to edit.

You need to edit the web.config file that is in the root of the
WebApplicaiton that you are getting the error on. Here's the easiest
way to find it.

1) Open Internet Information Services Manager on the web  Server

2) Find the Website which corresponds to the WebApplicaiton hosting
the Excel Services that you are trying to view. If you aren't sure
check the URL that you are accessing and compare it to the properties of
the websites.

3) Right click on the website and choose Explore. That will switch
the right pane of IIS manager to a Windows Explorer pane.

4) Right click on the Web.config file in that pane and select Open
With. Then choose your favorite editor.

There is one Web.config in the root of each Web Application. You need to
make these changes to each Web.config where you want to see the expanded
error messages.

Well, I did look for the web.config file and there are 20 plus files
with that name. How do I determine which file to edit.