Versions:

• Minecraft: 1.12.2
• CC: Tweaked: 1.82.2
• Modpack: FTB Ultimate Reloaded

Reproduction steps

• Start server
• Modify config/computercraft.cfg:http.S:blacklist
  Remove 192.168.* from blacklist
• Save config file
• Executecomputercraft reload on server
• Observe that the setting did not apply and the server has written the old config back to the file

Workaround

• Save and stop the server
• Modify the config as above
• Restart the server
• Observe that the config has not been reverted and the setting is applied.

🎉 Nice, thanks!