PSN Error (80028E06)

For the past few days, access to the Playstation Network via my PS3 has not been working, showing the message: "An error has occurred. You have been disconnected from the PSN. (80028E06)"
I then tried restarting my livebox, and I managed to connect to the PSN, but I notice that I can neither join nor play in multiplayer mode. Then, a short time later, I return to the same error message (80028E06). I then tried to re-enter my credentials (SSID and WPA) to connect to my livebox (as recommended by many forums), this works but the test then indicates that there is a connection failure.
I don't know if it's a problem related to the proxy or something else, but I want to emphasize that I haven't had any connection issues for the past 4-5 years with my PS3. So your advice is welcome.