Steps to reproduce (see Update below)
1. Create a clean instance of Ibexa Experience
2. Configure the second language, for instance, French (fre-FR)
3. Go to SiteSkeleton, create a Landing Page in English, publish it
4. Try to add translation

Result
The following exception is thrown. Please refer to the attached screenshot for the full stack trace.

Parameter "siteAccessName" for route "page_builder_content_preview" must match "[^/]++" ("" given) to generate a corresponding URL.

Expected result
Page Builder opens normally.

Update
This issue is also reproducible outside of SiteSkeleton. Steps:
1. Define 3 languages (eng-GB, fre-FR, ger-DE).
2. Create a Landing Page in eng-GB.
3. Translate it to fre-FR. Publish.
4. Try to translate it to ger-DE from fre-FR.