Roundcube + proxy = forbidden

Hello, I've installer my server, pretty much everything is working, but I have something weird. I have installed roundcube to read emails, and I also have a proxy. I can check my emails without proxy, but if I'm connected to my proxy, I can log in to roundcube, but as soos as I'm inside, I can't do anything, I have a modal that keeps saying "Server error (forbidden)".

I installed squirrelmail and everything was working fine (but I really prefer roundcube ^^).

Anyone has an idea?

Thx :)

Arvi