Hi,
I know the topic is old, but I had the same kind of issue last day with an apache2 webdav server.
Looking at the server logs, I saw the files were actually locked but unlocked very very quickly.
So, it is stupid but I just changed the default value of delay in Ganttproject > Project >webserver > open from webserver> server configuration. The default value of delay was [-1]. It was not suitable for my configuration. I put [120 min] and it is ok now.
Best regards and thank you for your work.
Marc