I am having the same problem and this is the screenshot:
https://imgur.com/a/Fws0X
I searched for a few hours and came across similar posts from dating back to 2015 until now. Most of them have no solution posted followed up. There is one guy posted the same thing on a Chinese forum. He mentioned he made a new install but this time he started the game first before installing TWoS. Others mentioned about the Generic Patcher, or setting DPI scaling to monitor.. But I have yet to successfully solve the problem doing any of that.
So far I've tried: Run as Administrator, high DPI restriction, clean reinstall, CCcleaner, installing on different drives, activating only the 2.0.0 mod without 2.1.2 updates, with and without Generic Patches, etc.
I have literally lost count of how many times I've reinstalled SH5 and TWoS.
As @THEBERBSTER mentioned above, I tried that as well. No success...
Apart from the "black corner" problem, also, the main menu looks a bit different:
https://imgur.com/a/9xq8E
I believe this is a UI problem caused by something I don't know.
Any help will be appreciated.
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Here is a quick collection of all the similar posts I've found:
http://www.subsim.com/radioroom/showthread.php?t=233898
http://www.subsim.com/radioroom/show....php?p=2537904
https://tieba.baidu.com/p/5133231169
http://www.subsim.com/radioroom/show...ck#post2536418
http://www.subsim.com/radioroom/show...ck#post2518492
http://www.subsim.com/radioroom/show...ck#post2512587
http://www.subsim.com/radioroom/show...ck#post2489574
http://www.subsim.com/radioroom/show...10703&page=518