There are 2 known situations that cause this problem and we found one workaround for each, but this problem is still being investigated.
If you have 2 or more screens connected in your device, try unplugging them before starting the game. This should fix the problem indefinitely, unless PokeMMO is set to open on a secondary screen again. If the problem happens again, repeat this procedure.
To confirm this, hold ctrl + shift + esc on your keyboard to open the Task Manager and click on the Performance tab. You should see which graphic card is being used on the top right of this window. You can also confirm if you have more than one GPU in this same window. They will be listed as GPU 0 and GPU 1.
If PokeMMO is running on AMD in your device and you also have another GPU, you can try switching it to this second one. To do so, follow these steps:
Finding PokeMMO's path
Changing GPU
If the problem persists after reading this article, please contact our support. Any feedback about these solutions or new information about the problem you might have is also appreciated.