To address your question, when an application runs in one of the external modes, a separate Process is started to control the target application and isolate it from the main Process. BPE maintains a connection to this Process, which it uses to send commands to and receive responses from the target application.
However, if the target application becomes unstable or freezes, this can bring down the separate Process and cause Blue Prism Enterprise to lose connection with it. As a result, the error message (or similar) that you report can be thrown.
Which mode are you using to attempt to attach to Edge? Are you able to attach to Edge using another/different mode(s), or is an error thrown each time? What version and bit-depth of BPE are you using (i.e. v7.4.1 64-bit)?
This Application Manager ("AppMan") error often originates from a mismatch between 32-bit and 64-bit installation, or between operating system/Blue Prism bit-depths and the application mode to which you are attempting to connect, so investigating if this occurs when trying different modes would be my first suggestion.
