There is a website solution in our company that uses SQL Reporting Service: in the solution there is a Reporting project, but this project is missing from solution. So I don't have access to these reports, but I know what procedures they call in the database. There is currently a production issue in only one of the reports, here is the error:
An internal error occurred on the report server. See the error log for more details. (rslInternalError) For more information about this error navigate to the report server on the local server machine, or enable remote errors.
I searched with Google and I found that I should look in the SQL Server Report Logs to see what is the issue, and I searched more but for all people experiencing this error it happens with all reports and not only a single report.
That error message only indicates that there was an error that is preventing the report from running; while it can indicate an instance-wide issue it's only related to the actual report that is being run, so can also be related to that report only.
You haven't said if you have searched the log file - this the best place to start. Any report exceptions will be logged here. You should check here if you haven't already - depending on your version check the files located in %programfiles%\Microsoft SQL Server\<SQL Server Instance>\Reporting Services\LogFiles\.
A few more suggestions to try and get more information - these all require various levels of access to the Report Server.
Enable Remote Errors
http://technet.microsoft.com/en-us/library/aa337165.aspx
This will mean that the actual error will be displayed instead of the generic error message you see, so you might get some further information this way.
Run the underlying Stored Procedure
Often there can be an error at the database level causing errors, e.g. a divide by zero error - what happens when you run the stored procedure? It's useful to capture the exact parameters being used through SQL Profiler so you replicate exactly what the report is doing.
Obtain the Report Code from the Report Server
You say you've lost the report code, but assuming a server-side report you can obtain these from the Report Server - this link has details for a couple of different versions:
http://sql-articles.com/blogs/download-export-rdl-files-from-report-server/
Debug report in Report Viewer
Similar to downloading the report definition above, you can open the server-side report in the Report Viewer application from the Report Manager website. This means you will be able to see the report designer in a similar environment to Visual Studio and check the report code, as well as running and debugging the report. Be very careful about doing this against a production instance - should only be done as a last resort.
Hopefully that gives you a few new options. It will be import to have admin access to the Report Server to allow you to perform all these troubleshooting tasks.