An Instance of Greenshot is already running

I have been encountering the same issue in 1.2.10.6, except my various attempts to resolve by uninstalling and reinstalling ended up with 4 separate launches on the list (confirmed by checking with Task Manager while the dialogs were up, and yep, 4 processes, all with the same executable path c:\program files\greenshot).

Only one startup entry listed when checked using Autoruns, only one registry entry to start, etc. What seems to have resolved this for me is closing Greenshot, re-running as Administrator, unchecking “Start with Windows” to get rid of the Run entry in HKLM, closing Greenshot, then re-running in my normal login and turning the autostart back on. This placed the autostart in HKCU instead.

To make sure we have multiple processes, and know which, I have added the process ID to the list of instances which are running.
Please do note, when you press the close button (previously ok) the Greenshot which tries to start is stopped... it will no longer be visible in the task manager.
So it would be best to look in the task manager while the popup is showing!
Maybe you can confirm that multiple greenshot process are started?

Here is the build:
https://ci.appveyor.com/api/buildjobs/wh594eyg12tj3ogi/artifacts/Greenshot%2Freleases%2FGreenshot-INSTALLER-1.3.0.5-ALPHA.exe

Now, this is important, as we need to know if the issue is realy with something already running or we are having false positives.
When there is something running, we need to see why.
If there really is nothing running, we need to see what is wrong with the detection.

Maybe the log can tell us a bit of what is going on, can you upload it to this ticket?

Here is information on where to find the log: http://getgreenshot.org/faq/where-can-i-find-greenshots-log-file/

I might ask you to try out a few versions of Greenshot where I make some changes especially for this bug.
Are you okay with helping us to track this bug?

For this I first need to know what Windows version are you using, and what .NET Version is installed...
I will need you to have.NET 4.5 or later, and Windows 7 or later.