Hi,
I followed your advice (disable one by one all my plans).
I discovered which plan causes the appearence of the "access deny message".
I also discovered what protection setted in that plan causes the message. It is in tab INTEGRATION, subtab EXTENSIONS, field "Components which access depends on this plan". There I selected a component; if I unselect that component the message stop to appear.
Now... why the protection of that component by the plan causes the appearance of the "access deny message"? This message appeares indiscriminately in all pages in my website, also in the those where there is nothing related to that component (I also tried to disable all modules). And I repeat that the message sometimes appears and sometimes not... without any particular regular behaviour.

So... any idea to fix this problem?

Thank you very much!

I forgot to say two important details:
1. The same plan protect two components: one (if selected) causes the appearence of the "access deny message", the other one (if selected) doesn't cause the same problem.
2. The component that is protected by the plan and that cause the problem has a strange particularity: it appears 4 times (with the same name) in the list of extensions. If I selected only one of them, after saving I find that they are all selected.