Do you have any endpoint security software running (Symantec for example)? I'd ensure it's not stopping a process.
Secondly, as weird as it may sound, check your PC to ensure it's using the appropriate graphics card when it runs D2R. I had similar issues months back and it turned out i had a cable loose on my graphics card.
Here's what Blizzard well ask you to check:
Hello, In an effort to resolve these unexpected situations with the connection issues in your Battle.net Game, we've developed this extensive troubleshooting so that you can get back into your game as soon as possible! To resolve any outstanding situations, we ask that you try the following steps outlined in these articles: 1. Restart Your computer and Router/Modem: Sometimes, a simple restart of your networking equipment can resolve connection issues -
https://support.blizzard.com/article/000031851 2. Check Firewall/Antivirus Settings: Make sure that your firewall or antivirus software isn't blocking Blizzard's games or services. You may need to add exceptions for Blizzard's executables or ports -
https://support.blizzard.com/article/000007842 3. Disable Background Applications: Close unnecessary background applications, as they might interfere with Battle.net Games. Check for any resource-intensive programs running in the background and close them before launching the game -
https://support.blizzard.com/article/000023848 4. Flush your DNS cache and reset your network configuration. You can do this by opening the Command Prompt (on Windows) or Terminal (on macOS)
https://support.blizzard.com/article/000082450 5. Reinstall the Game: Uninstall the Game, restart your computer and reinstall the game. Corrupted installation files can lead to various issues, and a fresh installation might resolve the problem. 6. Wired Connection: If you're using a wireless connection, try switching to a wired connection by connecting your computer or console directly to your router using an Ethernet cable. This can help rule out potential wireless interference issues. Should any of the above articles not solve the on-going situation, please consider contacting your Internet Service Provider (ISP) for further troubleshooting, as they could provide more insight into these matters.