Vlad

It is a limitation in snap package configuration. It is a security feature of snap to run a program in a sandbox so it is possible to limit its impact on the system. Apparently, the current configuration of the snap with Freemind that you are using limits file access to the home directory, or so. Here is a thread that discusses that problem: https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1643706

I had the same problem. The root cause in my case was that Freemind application opened a modal window that was hidden behind other windows, sometimes even on another display when two monitors used. Naturally, Freemind does not response on keyboard and mouse clicks until the modal window is dismissed. It might be tricky to get the modal windows in focus. You can try to minimize all the windows (WIN+D in Windows) and then bring Freemind to the front.