Article categories sometimes showing empty

Hello!
We are using IssueM to publish articles on nafnleysa.org. Our top level menu are article categories to which articles are linked. When I open a category, a list of articles assigned to it comes up 80% of the time as it should. The rest of the time, no articles come up and the page says “We can’t find what you are looking for.” I tried it in different browsers but for example in Chrome, it sometimes works and sometimes doesn’t so it doesn’t seem like a browser issue.
Do you have any idea what might be causing the problem and how to fix it so the articles always come up?
Thank you!
Michaela Svobodová

https://ww.wp.xz.cn/plugins/issuem/