Like Apache, LSWS performs internal redirect if the target URL is a relative URL without leading "http://". During the internal redirection, no duplicate URL with same parameters is allowed.
To avoid the internal redirect, you can change your redirect url to a absolute URL.
Please turn on debug log by changing "debug level" to "high" under "Server"->"General" tab, you will find out what is happening. If need more help, please post lsws/logs/error.log here.