Hi, the Gnome project uses the gio mount command to manage network shares in their gvfs file system. There you cannot specify a mount point anymore like with the normal mount command. Instead the mount points look cryptic like ‘/run/user/10/gvfs/smb-share:server=192.168.0.0,share=homes/blabla’. And here is the problem: LibreOffice cannot write on such a path - I suspect because of the special characters, since it works when using standard mount points. Is there a way to fix this? Thx a lot!
And you are sure that these mounts to gvfs are done read-write and not read-only?
Of course 
With other programs I can write without problems.
I just found out that these special characters in the path are not a problem for LO! If I create such a folder on a normally mounted share, then LO works just fine in there. So basically LO can read files, create files, but cannot open files for writing on GVFS.
It seems that you need another step to make those mounts accessible by normal path syntax. You make a symbolic link.
That I gathered from this page.
Yes, I have s-links in my home folder to the long mount points – but this doesn’t help at all unfortunately
I found in another forum that the Snap installation would be the problem. So I removed it and installed LO manually - the new version 7. Now everything is fine. Thanks for helping!