I have also a multilanguage site English/Dutch/French. All pages in Dutch give 404, page not found, when editing in frontend mode. the page in English is ok.
This happens after upgrade , the last months , this works normally. Today i've tried with also upgrade the language packet to the version (3.9.28) but still the same problem.
When i change the language in page builder from dutch to english then i can edit the page in front-end but that is not the final solution.
Last joomla version - helix ultimate template - everything worked well, till now
Best regards,
Ignace