Getting a "Network path was not found" when trying to open a database connection. Why is that?

We've got an older ASP.NET Web Forms app. I just replaced 8 Crystal Reports .rpt files and 1 .aspx file. Now, a file I did NOT replace is complaining with the following error message:
Server Error in '/ClientSystem' Application.
<div align="center"> </div> The network path was not found
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.ComponentModel.Win32Exception: The network path was not found Source Error:
 
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