Hi Nicola. Several people who had used previous versions of MuseScore have reported weird issues like this. In each case, it turned out to be a configuration issue. I’ve seen this before. Newer MuseScore versions choke on configuration files written by older versions. Please try a factory reset. To do that, open the Help menu. Revert to factory settings is the last option. Restart MuseScore after selecting that option.
Warning: this will erase all of your customizations, recently opened files list, and plugins. It’s a pain, sorry.