Lotus Notes an error was encountered when opening a window
Do not reboot your computer ! All you have to do is to terminate the ntaskldr.exe process.
This might happen when hard closing Lotus Notes, for example by terminating the process, or asking Windows to do it when the program does not answer, you might face this issue at the next Lotus Notes start.
Simply go to the Windows Task Manager (by pressing CTRL+ Shift + Esc, or by pressing CTRL + ALT + DEL => Start Task Manager).
There, in the processes tab, find the process called ntaskldr.exe (or nnotesmm.exe or nlnotes.exe), and click on End Process.
Validate again that you do want to close this process by clicking End Process in the confirmation box.
You can then open Lotus Notes again from the Windows Start menu or another desktop link / URL ...
... and you're back in Lotus Notes logon window!See more details on IBM Error: 'An error was encountered while opening a window' when launching Notes
Lotus Notes error
When encountering a Lotus Notes error, and an action cannot be done, always start by trying to kill the LotusNotes process, by opening the Windows Task manager.
Then, find the ntaskldr.exe process, right click on it, and select end process.
This will kill the Lotus Notes process, and you can now try to restart the LotusNotes application and see if the error is happening again or not.