I recently downloaded Fiddler Classic and now that is downloaded, my Slack app does not connect unless the Fiddler tool is up and running. What settings do I need to change in order to have Fiddler closed and ability to use the Slack app?
From the provided details, it looks like that when Fiddler is turned off, the OS proxy settings are probably not reset (to use no proxy or the default system proxy). If no proxy is set, your Windows settings should look like the ones in the fiddler-no-proxy.png image(when no Fiddler is running). When Fiddler is running, the proxy settings should have set 127.0.0.1:8888 (default proxy for Fiddler), as shown in the second image (fiddler-proxy-classic.png).
Try resolving the issue by making sure that Capture is turned off, then closing Fiddler, closing the Slack client, and restarting the Slack client. If the problem persists, then open the Proxy Settings OS window and make sure that your settings are reverted (the Fiddler proxy should not be present when Fiddler is turned off).
Apart from the above, it is known that some network tools might interrupt the Slack client, as stated in the official Slack documentation. Check this page for additional information and instructions.