Web server rejects utm_expid
Your server doesnt support added query arguments in urls
If you receive the above message while validating your Google analytics experiment then try to check the following cases:
1) Open the link which shows the error while validating and do "view source". Check if you can locate the experiment code in the file. If yes, then proceed for step 2. If you are not able to see the experiment code then the code is not mentioned at the correct page.
2) Now add some url parameters to the url. Suppose the url that you are trying to validate is "www.testurl.com", then add few parameters in the url like "www.testurl.com?utm_expid=test&utm_referrer=test". Now see if the page is coming up properly and not going to a 404 or page not found error.
Now, do "view source" and check if the experiment code shows up on the same page as well.
I hope the above two cases will help you debug the issue.