https://github.com/greenshot/greenshot/releases/tag/Greenshot-RC1-1.2.10.3

Disabling windows update, and being online, would not be the best course of action... but that is up to you

Thanks for testing, so this fixes the issue which is GREAT!

I would advice you to go to a release version of Greenshot, as soon as 1.2.9 gets an update (if, than this fix is in it), or 1.3 is released..
I cannot guarantee that everything works...

If you have a lot of issues, please try to go back to 1.2.9 first.

P.S.
If I think about this ticket, I will comment that a newer version is in it... but don't count on it, we have so many things to think about!

Yes, I checked and it appears that I have 4.0.something version of .NET. Maybe installer did not updated it because I turned off WinUpdate. I installed .NET manually from provided link and all seems to be working fine now. I successfully logged in on imgur and uploaded screenshot. Thank you for your help!

It seems that you do not have .NET 4.5 (or later), which is double weird as the installer should have detected this and windows update should have updated your .NET version!

But as the installers changes are is new, there might be something missing.
I also should have changed the executable not to accept anything lower than 4.5..

First let me know if you have any reason not to install a newer .NET Framework, because for us there is no way we will make 1.3 work on an older .NET version. (You can install it via: https://www.microsoft.com/en-us/download/details.aspx?id=53344 )

We can probably fix 1.2.9 for this, but I rather would go forward.

Installed this version, tried to open selection in editor, got this

Software version: 1.3.0.6-ALPHA-ace0aecd7d9fe3e43e13903739ec25badd7a88bb (64 bit)
.NET runtime version: 4.0.30319.239
Time: 2017-02-09 14:53:18 +04:00
OS: unknown (x64) 6.1 build 7601 revision 10000
GDI object count: 31
User object count: 21

Exception: System.MissingMethodException
Message: Method not found: 'Int32 System.Environment.get_CurrentManagedThreadId()'.

Stack:
at GreenshotPlugin.Core.WindowDetails.<GetVisibleWindows>d_130..ctor(Int32 <>1_state)
at Greenshot.Helpers.CaptureHelper.RetrieveWindowDetails()
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()

Configuration dump:

And after that Greenshot crashed.