Ok, somehow the issue suddenly vanished and all the posts are showing. I asked Pie Calendar support and they wrote this:
Usually this is caused by permalinks needing to be refreshed. This is common after creating a new CPT. You can go to Settings > Permalinks and re-save them twice and the issue should be resolved.
I am not sure what you mean by choosing a post type slug. I created the post and then I am clicking on the permalink available in the top of the post. I installed your plugin to use the pie calendar plugin (they recommend doing that on their site), and the post link is also not working in the calendar.
