After installing the gvfs-1.48.1-150400.4.3.1 update on Leap 15.4, I upgraded Samba to 4.15.8+git.500.d5910280cc7-150400.3.11.1. I am now able to connect to my NAS again with the latest version Samba. Looking at https://bugzilla.opensuse.org/show_bug.cgi?id=1198718#c8, it appears that this bug might also be fixed on Leap 15.3 with the gvfs-1.42.2-150200.6.3.1 update.