Hi,
I had a bug « a bit similar » : I could not save the Onlyoffice app settings in Nextcloud with NC and OO in 2 docker containers in the same machine (network mode bridge). My error message was a bit different : « bad health check » or « error while downloading the document being converted ».
I could resolve it with adding the local IP and Domain names of both nextcloud and only office in the « trusted domains » array in NC config.php : [Onlyoffice] Error while downloading the document file to be converted ou encore ici : https://github.com/ONLYOFFICE/onlyoffice-owncloud/issues/181
So now I can save the settings !
But I still can’t open OO documents : when I click on a .docx doc in Files for example it says « onlyoffice unreachable ».
I also found a page that could help : https://api.onlyoffice.com/editors/nextcloud
@edvinkuric and @syrosh what did you put in the 3 server fields « external doc server », « internal doc server » and « internal NC server » ? It could help to understand the pb