The page for each event doesn’t work

Hello,
I struggle with a problem for months on the website of a customer, and I’m now with a lack of ideas… Could you please help?
Here is the bug: the pages of each event don’t show up anymore… sometimes (I didn’t find any pattern until now, unfortunately).
– If I disable the “WP Manager” plugin and then reactivate it, it solves the problem (it means that I can’t run another test and just have to wait the next time that the bug appears. Sometimes it’s almost each day, sometimes it works for a few weeks 🙁 )
– I tried to desactivate each plugin one after the other and I found out that disabling the Solid Security plugin (ex- iThemes Security) solves the problem. But when I activate it again, the bug doesn’t appear anymore. So I can’t continue to search the rootcause until the bug appears again 🙁
– I did some researchs, searching for a maybe-known problem including both of those plugins, but the only thing I found is the XSS vulnerability of the WP Manager plugin. It seems it appeared some months ago and there is still no patch to solve this in the new updates of the plugin. I don’t find any other issue online.
My customer was very nice until now, but I can’t let the situation like this forever: people can’t register to its events when the bug appears and none of us has seen it. Could you please help?
Thank you in advance for your help!

The page I need help with: [log in to see the link]