Cannot find application RSLinx Classic. Start application? error when RSLinx Classic is running

When I start my client application (RSView32, RSTrend, Excel, Access, and so on), I get an error

Cannot find application RSLinx Classic. Start application?

, even though RSLinx Classic is already running.

You may be using RSLinx Classic Lite instead of RSLinx Classic. If the title bar says RSLinx Classic Lite, your system may not have been able to find the copy protection and ran RSLinx Classic Lite, which is unable to use DDE. Either insert your key disk or move the activation (copy protection) to your hard drive. Refer to the Getting Results with RSLinx Classic guide for more information.