Project is already open for file <Quartus II project file> in another Quartus II software application

Description

You may see this message when trying to open a project in the Quartus® II software. This may be caused by a lock file that prevents you from reopening the project. If a previous Quartus II session terminated unexpectedly, this lock file may not have been automatically deleted.

To resolve this problem, ensure the project is not open in any Quartus II session and manually delete the lock file db/<project>.quiproj.mtx.flock.