Hello together

I installed WPForo on my website and integrated all the themes I want to have. Unfortunately, I am a beginner, I do not understand the following problem at all. If i click on Members or Login, I get an error message.

https://trailtalents.ch/participants/    (Not found)

Do I write but in the link: https://trailtalents.ch/ index.php/participants/
the page works. Can someone help me how to solve this? I do not understand the index.php at all. 😓 

Thanks a lot
Phil