Strange NFS problems with recent Linux kernels and FreeBSD

Hi all,

I am running a FreeBSD (5.4-RELEASE) NFS server, sharing $HOME and some
other directories.

I used Debian Sarge with Linux kernel 2.6.8 so far for all the NFS
clients - and everything worked flawless.

Since a while I am upgrading these clients to Debian Etch and therefore
to Linux kernel 2.6.16 (Debian Kernel) and to the latest NFS packages -
and now I have the following strange problem :

Whenever a file on the NFS server belongs to a different user but is
group-writeable(!), I always get "operaton not permitted" when trying to
write the file (lets say a simple textfile edited by vi) - and even
worse - the file has a size of 0 bytes at the end !!

If the file belongs to the user itself everything works like expected.

Again, with kernel 2.6.8 from Debian Sarge everything works fine. I now
tried Linux kernel 2.6.15, .16 and .17 - ending up in the same error
described above.

I cannot see any warnings or error messages neither in the syslog of the
FreeBSD machine nor on any of the NFS clients.

Any ideas what the problem might be?

Thanks in advance, Joerg.