We found the issue in Toolset > Post Types > Firmy > Options > Rewrite > Use a custom URL format. There, you inserted /.
Please check the following screenshot: hidden link
If you instead use "Use the normal WordPress URL logic" then all works fine. Note, if of course also works fine if you rewrite the URL, but you need to use something else than /, for example, a string, to be prepended.
The reason it works for some posts but not others is that the user uses cache and old permalinks are stored for the posts that work, but not for those which do not.
So instead of using /, you should use a slug/string.