Minecraft stuck on Terrain loading screen problem

A friend of mine can't use Minecraft anymore after she installed Optifine 1.13.2, suddenly the game said that two instances are running and thus she could not start the game. Before that she always used Optifine for Minecraft version 1.12.2 and everything was alright. She tried to close the second instance through Task-Manager, but it was to no avail. She reinstalled Minecraft deleting both folders, the folder in Program Files 86x and the .minecraft folder appdata leaving no traces of Minecraft behind, downloaded it from the official site, installed it without any modifications and still was unable to start neither single nor multiplayer due to the endless "Terrain loading" problem. She also tried to set the view distance to 6 under video settings options, but this did not help either. She ran Minecraft as Administrator and right out the Program folder, but the problem remains. Right now we don't know how to solve the problem. I don't understand what could cause this error, that was never there beforehand. All that is left is Java, but she says she already has reinstalled it. Then it could be that the registry is corrupted, but I don't know a thing about Minecraft and registry keys and if they really can cause such a corruption. Maybe someone here has a solution.

Thanks to your suggestion. We tried very many things for hours on end and I read all kinds of logs, but nothing of substance could be found. In the launcher log for instance, I discovered that the game didn't load further after having have loaded the sounds. My friend and me kept on trying plentyfold of things, some of them also inspired by a thread here on aternos. We, downloaded Malwarebytes and cleaned the system, disabled autostart of Discord in Windows and allowed Java and Minecraft to go through the Firewall. I don't know exactly wich of these things did it, but now it functions.

-SOLVED-