I do not understand why but now everything seems ok.

Possible that the forum will be adjusted by yourself?? 

Have you fixed whatever the cause was? They are all loading up perfectly for me?

The first thing I thought is that since the site there are three static pages with the same name, maybe there was some conflict, but I do not think is possible because the pages of the forum are in a different directory (ie /forum /) .

It maybe just needed a flush of the WP rewrite rules to understand the permalinks. So – is all good now?

No! The problem is back 

Still perfect from my end. Have you tried clearing out your browser cache?

For example, the problem is not there now, but every time I get.
Maybe in a few moments wordpress gets confused?

Maybe I should change the slug to differentiate the forum from the pages?

Always worth doing to avoid possible trouble.

In my admin panel of the forum I have seen some reports that are error log.

Could explain the problem?

Here is the screen.

I really do not think so although it is a strange error to be getting.
Do you want to correct it if I give you the code fix so we can see?

Yes, of course. Thank you.

OK – here we go.

The file you need is /forum/content/classes/sp-forum-view-class.php

The line of code (line 454) is quite a long way down. If you find the start of the function –  sp_forumview_build_subforums() – then the line we need is not far above that.

It currently reads:

if ($f[$forumid]->children) {

and you can change it to:

if (isset($f[$forumid]->children) && $f[$forumid]->children) {

be interested to hear back from you.

Then: I copied and put the code in the specified line.
I also reported the slug of a forum of the previous version (the same as the page).
I tried a couple of times and seems to work.

For safety during the day I will try again every so often to see if the erroroccurs again.

For the moment everything is ok, thanks!

Curious. Please do keep us informed.