I am using the Windows version of gtk-gnutella on a Windows 10 installation. Recently it stopped working. It will not start and I have no idea why. I tried uninstalling and reinstalling the application. Does anyone have any ideas on why it wouldn't start?

One thing to try is right-clicking the GTK-G app and choose Run as Administrator.
Failing this you might also try changing the program's compatibility mode to see if any such changes make a difference.

Note: I am not certain where GTK-G keeps the settings folder on Windows.
With uninstalling, did you also remove the program settings folder, it's often an option when uninstalling apps? That can sometimes fix certain issues such as if a file is corrupted or data within is corrupted.
START menu > type/copy-paste %appdata% (In theory this is where the settings folder should be.)
You should find GTK settings folder within. Though I have a suspicion GTK may hide it. In which case you would need to turn on Show Hidden Files & Folders in system settings. https://support.microsoft.com/en-us/...1-9b7f9840fdb2
But if you do this, then re-install the app because GTK-G pre-installs important data to allow the app to start and connect smoothly.

Thanks for the tip but deleting the ...\AppData\Local\gtk-gnutella folder did not fix it. Any other ideas anyone?

If you are registered at Sourceforge you could potentially post a new ticket here.

AFAIK the Windows dev is no longer active. But the main dev might be able to help.

Edit: Have you installed any security software or updates in recent times? There is a small chance a security app (possibly 3rd party) is preventing GTK opening. This appeared to be a relatively common occurrence several years ago with p2p apps being considered non trusty by such security apps (their devs were anti-p2p file-sharing.)

Also keep in mind afaik GTK-G might not be a properly signed app, so you might have inadvertently made a selection when attempting to open that has become default behaviour to disallow.

You might also try running the app from a different log-in account.

You should exhaust all possibilities before posting a ticket at Sourceforge.

I figured out that it was Comodo Firewall blocking the app from running. Even after uninstalling Comodo Firewall it was still blocked. No problems now with the app.

Thanks for the feedback and good to hear. I've had very similar issues with a 3rd party firewall on Windows before dumping it.