Invalid URLs.

On sqwebmail from Debian Sarge:
When I am in the folders list, I have URL such as:
http://mysite/cgi-bin/sqwebmail/login/knoppix.authdaemon/B8F4914528615600F0D
25DBEB3FCE72A/1088093566?&form=folder&folder=INBOX
to access my INBOX.

I have my Apache server hidden behind a Pound reverse-proxy
that filters illegal URLs.
And the above URL is typically an illegal URL:
the argument list is not supposed to begin with a &.

It is ok for Apache. But not for Pound: it returns a 501 error
and insults me.

Is it a known issue of sarge version? 
Or an misconfiguration on my side?
Can anyone reproduce it?
Was it corrected in future versions?

Any idea is welcome.