The symptoms I see look remarkably like this bug.
If you follow it all the way through, it leads to this being filed.
A very interesting read. Especially the very last post on the 2nd link above.
I don’t know what happened after that. Maybe someone fixed it and it’s regressed(?)
The gist is that pcmanfm-qt is not sending the gio (smb://) type paths to libreoffice. Instead it’s sending the /run/user/1000/… paths which libreoffice is not handling well.
I have not tried the workaround script given in that last post but it’s probably ok, because I tried
libreoffice smb://… in a terminal and it worked.