hi @el_lobo sorry I missed your post for long time. I’m not an expert with collabora-built-in but I learned it doen’t follow Nextcloud mechanics like overwrite* settings on config.php but relies completely on reverseproxy headers like X_FORWARDED_*
for me it looks like your built-in CODE doesn’t know how you access the system ( search for X-Forwarded-Host) maybe this post and thread with working apache config heads you into right direction: