Very Slow connection

Hello everybody, I am experiencing a very strange behaviour with my Raspberry.
I have an Rpi2 (tried also with an Rpi3) with Hifiberry Digi-pro (tried also with Digi notPro), linked to a TP-Link TL-WR702N.
The installation goes right, but when I try to connect via WebUI to the IP provided, the browser seems stucked and take a lot of minutes to load the Volumio homepage, and whenever is visualized, all the tabs appear like they miss some “mask”: they look like COMMON.TAB, SEARCH.TAB, etc…
I tried both my Rpis and with different browser from different devices (Safari-iPhone, Chrome-iPad, Firefox-PC) but nothing changes.
On the other hand, if I use a very old version of Volumio (1.55) everything’s work properly, but the overlay for the Digi-Pro doesn’t work.
Is there something I am missing?

Thank you very much!