Hi Dima!
Sorry for leaving you hanging for so long with this.
After we have consulted with the relevant team we concluded that the above behavior isn’t a bug,
But misuse / misunderstanding of a very complicated subject and of the feature.
Server-Side-Rendering works only for first page to load. All navigations inside the site are on the client-side only.
Since wixLocation is a front-end method all the rendering of the following pages happens on the client side.
Therefore the backend has no access to the wixLocation.query and that’s why we get an error.
I hope thats clarifies the subject of the thread.
Let me know if you need more information!
Doron.