goldhat
Many thanks for your answer. After a few days of trying different solutions I realized that the problem was another Plugin, PageBuilder from SiteOrigin. Any time i wanted to update the quiz,I have to disable the plugin, make the changes and activate it again.
In general, it seems that any problem related to wp-Pro-Quiz is because another plugin, so I recomend to disable all plugins except that one and then activate one by one until you find the one that creates the problem.
