thanks for doing some research, but what should the solution on stackoverflow bring? That entry is extremely old... Greenshot was working fine for a longer time and recently Imgur must have changed something.
Anyway, this is a duplicate of BUG-2235, where I already presented a solution. The 1.2.10 release will be available in one or two weeks.
You can not upload to a specific imgur account.
If I start the upload I get the screen found in
.
I tried to find the issue by my own and what I got was the error shown in
.
I also did some research and the solution lies here on stack overflow