here are two pictures with describing text, we are several persons having the same problem.
I cant find anything in the logs folder under nodebb with errors, or anything under /var/log/.
I am running with a MongoDB backend and followed the instructions on how to install nodebb with MongoDB from your readme page. I therefore cloned your github repository directly, so I'm on dev.
I think this is a problem with the bootswatch skins, can you change to lavender and see if it happens? There was another topic about this will try to find it.