I get blank pages when using my custom template

Hi,
I have installed the plugin and made sure that all Anspress pages have the shortcode. I created a “Forum” page to use it as a base page and pasted the anspress shortcode too. But when I try to load the forum page, I get the famous blank page.
I activated the WP debug mode but I have nothing in my debug.log file. I also installed the debug bar plugin, and nothing.
I installed the Health Check plugin, same. I removed all unused plugins, and made a troubleshooting to see if there is a conflict with the activated plugins: nothing. The only thing that makes the Anspress plugin work is switching to a default theme.
Can someone help me solve this issue please? I read every post in github and Anspress cummunity, I read the FAQ, the installation guide, but couldn’t find any solution.