This answer is from Tomas Herceg at Dotvvm:
There are two things about the URL:
1) DotVVM uses routing for all pages, it is not possible to access the page using the path to the dothtml file.
You need to register a route in the DotvvmStartupcs file (https://www.dotvvm.com/docs/tutorials/basics-routing/latest), or alternatively you can auto-discover routes.
2) Then, the page should be matched by the routing mechanism. If it's not, I would suspect the / characters in the query parameter - they should be URL-encoded.
3) If you want to make hyperlinks from one page to another, I would recommend using the and composes the URL for you.
https://www.dotvvm.com/docs/controls/builtin/RouteLink/latest