Voice of the customer Survey Link is broken

We have a survey that was previously working but the live link has broken.

If you click on Preview, it gives the URL for:

https://crmemeavoc1runtime.crm4.dynamics.com/xxx-b3cc-xxx-9125-xxx/pmo-feedback-survey_preview/preview_xxx

In Test it gives the URL for:

https://crmemeavoc1runtime.crm4.dynamics.com/xxx-b3cc-xxx-9125-xxx/pmo-feedback-survey/Test_xxx

But if you click on the "anonymous link" field the URL is:

https://crmemeavoc1runtime.crm4.dynamics.com/xxx-b3cc-xxx-9125-xxx/pmo-feedback-survey 

but the URL then displays an "HTTP 400 error: its not you its this link (it appears to be broken)"

Can anyone suggest whats going on here?

thanks