Just like the error suggests, you need to check your antivirus or firewall. Most likely, the connections to the master server are blocked. You can simply fix this by adding the Gramblr executable to the Exclusion list.
Additionally, you could reinstall the application. This time use Run as Administrator to make sure that all the permissions are set properly.
I also recommend following the instructions on the official website:
- Try the reboot button at the top left in Gramblr (Mac: Reboot your computer).
- Try closing Gramblr in the task manager (Windows).
- Download and run the newest version.
- Install OpenDNS
- Allow outgoing connections to app.gramblr.com:4344
- Sign out of your Gramblr account, rename the file C:\Program Files\Gramblr\gramblr.exe to gramblr.exe.bk, extract the newly downloaded one there, and end the Gramblr task in Task Manager and re-open it.