Why can't I create any operations all of a sudden?

I have to close the program and restart it, then it works again. Is this is a known error? The file is located on an encrypted drive. (Boxcryptor) Perhaps this is significant.

Please post the log file which is produced when it happens. If you are on Windows then the log file is ganttproject.log which sits in your user home directory (which is usually C:\Users\<your user name>)

Hello Dmitry.

at the moment I can not repeat the error. As soon as I have a log I will report again. For now, thank you for your answer.