reverse proxy replaces square brackets in request

[Isaac Hailperin]

Hi,

I have a redirect loop problem with a website that gets reversed 
proxied. It looks like the following is the cause (which I don't know 
how to solve)

Inspecting the site with httpfox, the request my browser sends looks 
like this:
https://www.acme.eu/acm/admin/gui_call.php?Object=admin@GuiAdminStartpage&Params[gui]=&action=&no_subtitle=1

My nginx log tells me this:
GET
/acm/admin/gui_call.php?Object=admin at GuiAdminStartpage&Params%252525252525252525252525255bgui%252525252525252525252525255d=&action=&no_subtitle=1
HTTP/1.1"
  HTTP/1.1" 301 486 "https://www.acme.eu/acm/ui/" "Mozilla/5.0 (X11;
  Ubuntu; Linux x86_64; rv:13.0) Gecko/20100101 Firefox/13.0.1"

(This repeats over and over until firefox detects the loop)

To me it looks like somehow nginx changes the square brackets around 
"gui" into "252525252525252525252525255b" and 
"252525252525252525252525255d" respectively. I assume that because the 
script gui_call.php gets wrong parameters, it redirects to /acm/ui. 
/acm/ui calls gui_call.php with wrong paramters, and so on.

If my interpretation is correct, how can I stop this? If not, what is 
going on here?

Isaac