Cannot open LibreOffice files on Samba share concurrently on Debian Linux - File locking issue

We have a Samba file server and use Debian Linux for both server and clients. Everything works fine. Except for one thing: If someone opens a LibreOffice document (mainly Calc / *.ods), nobody else can open the same file concurrently, not even "read only" or as a copy. If I try to open such a file, LibreOffice says it is locked by a certain named user or by an unknown user (the latter happens if I rm the ~lock. file). Then I can choose if I want to open the file read-only or if I want to work with a copy of the file. That's fine. But none of both options work! If I choose "open read only", nothing happens. If I choose "work with a copy", an empty Writer document opens up (although I tried to open an ods file with Calc). Next thing I tried: I can't even run md5sum document.ods! It says permission denied. So it's not just LibreOffice that cannot open the file for concurrent read access. LibreOffice seems to put an exclusive lock on the file when it is opened for editing. Nobody else can open the file until the first user closes the document. Any ideas what I could try? Preventing concurrent write access is reasonable. But why does it also block concurrent read access? And how can I disable that? getfacl output on the file server looks fine so far.