Thanks for reporting this! Please submit an issue on GitHub to bring this to the Arduino developers' attention:https://github.com/arduino/Arduino/issues
Has there been any progress on why this error occurs or how to fix it?
Please do this:Start File Explorer (AKA Windows Explorer).Open the Arduino IDE's installation folder (usually C:\Program Files (x86)\Arduino)In the folder listing of the Arduino IDE installation folder, hold the Shift key while clicking the right hand button on the mouse.Click "Open PowerShell window here" (or "Open command window here" if using Windows 7). PowerShell (or cmd if using Windows 7) will now open with the current directory set to the Arduino IDE installation folder.Type ./arduino_debugPress Enter. Debug output should now be printed to the PowerShell window as the Arduino IDE starts (or fails to start).Wait for the Arduino IDE to finish starting, or failing to start.Switch back to the PowerShell window if the Arduino IDE window took the focus.Right click on the title bar of the PowerShell window.Click Edit > Select All. This will select all the text in the PowerShell window.Press Ctrl + c. This will copy the contents of the PowerShell window to the clipboard.In this Arduino forum thread, click on the reply field.Click the </> button on the forum toolbar. This will add the forum's code tags markup to your reply, making it easier for us to read.Press Ctrl + V. This will paste the output between the code tags.Move the cursor outside of the code tags before you add any additional text to your reply.Click the "Post" button.Hopefully that will give us some information we can use to help you with the problem.
If you want progress on it, please provide the information I requested previously:Those instructions are Windows specific, because @leexande said they were using Windows. If you're using a different OS, the procedure is essentially the same, but you won't have an arduino_debug.exe, that's Windows-only. On other OSs, just run the arduino executable from the command line and post its output.
I have just installed the 1.8.12 version in the portable mode. For sake of sharing information the IDE loads up and there appears no issue as it opens my last sketch that i closed on the 1.8.11 version.
If I compile with 1.8.12 and forget to include a include of my own files there is no error thrown.