I've experienced a similar problem before so I'll respond in case you have the same issue.

In the dashboard you'll find a list of words that will be excluded from URLs. I've found that sometimes I create a page and it strips out parts of these words from the URL.

Here's an example. Let's say the word "how" is in the excluded list and then you create a new page called "however-hypothesis", then I have found that the page might be called "ever-hypothesis" but the link to it is looking for "however-hypothesis" so it's broken. Renaming the page fixed it.

This isn't necessarily your problem but it's a place to start looking. Look at the link your browser is seeing by hovering over the link and see whether it is correct.

I have deleted all the excluded words from my list and I don't get this problem anymore.

Good luck,
Mike

PS my issue was on 5.6 not 6.7

Well... Everything seems to be back in place... I don't understant what happened here but everything is back, so Yeay!

Thanks for your reply, I've look to the exclude words, but no issue here for me. No issue at all for now.

The issue is back with the new 5.7.5.2 update on three pages. Last time it kind of resolves by itself (I guess), and I still have no clue on how to fix it.

@Nomys

Please include screenshots of the problem pages.

Are you using a custom theme?

Do you have any errors in your browser console?

Thanks for your reply !

Screenshots are in admin mode.
I'm using a custom elemental theme. No errors on Firebug.

@Nomys

If you create a guest admin account and send me the details in a private message, I can take a look.

@Nomys

The concrete5 toolbar is available on the pages you took a screenshot of.

I believe you are not seeing the toolbar because of caching. While developing your site, it is recommended to disable caching.

- disable all caching
Dashboard > System & Settings > Optimization > Cache & Speed Settings
- clear your cache
Dashboard > System & Settings > Optimization > Clear Cache

That's strange.

I've already cleared the cache, I've even deleted the content of the files/cache folder...

Disabling all caching don't change anything. I've tested all that on firefox and chrome (and on an another computer).

On the Deliverance page, do you see any pictures, like it should ? Or do you see the admin interface but no images and no style on textblocks ?

edit : I've tried with the account I've made for you, in case it was my account the problem, but it didn't change anything.

@Nomys

I am attaching screenshots of what I see.

Are you hosting the site on a personal computer?

Ok, so everything looks normal on your side, do you see the same when you're not loggin ?

I'm not, it's all on a distant server not personnal.

I guess that the pictures are too heavy, I will put some smaller ones to test.

edit : I've put smaller images, no timeout error anymore, I'll check the server, but for now everything seems back to normal (again).

Thanks for your help MrKDilkington !