It is very likely, assuming Samba worked before on the host, that updating another port or package replaced
libgcrypt.so.15 with a later version, while the
smbd binary was linked with that specific version. Check if you have a
libgcrypt.so symbolic link pointing to a a different numbered version.
In that case, assuming that the newer version is backward binary compatible, you should be able to to work around this by creating a symbolic link from
libgcrypt.so.15 to the newer version you have. It is however recommended to rebuild Samba with the newer version instead, in case there are any differences in the include files of the library.
Another possibility is that the package containing
libgcrypt.so was uninstalled (due to a new conflict dependency or deprecation), but if Samba was installed from a FreeBSD port, this should have issued a warning.