Xauth/X11 ssh forwarding errors with .Xauthority file not writable

When logging in with ssh -Y remotehost I get the following error message:

/usr/bin/xauth: /home/hlovdal/.Xauthority not writable, changes will
be ignored

And xclock and similar fails to start. There is nothing wrong with the permissions
of the .Xauthority file despite the error message.

(hlovdal) remotehost:~>xclock
X11 connection rejected because of wrong authentication.
X11 connection rejected because of wrong authentication.
X11 connection rejected because of wrong authentication.
X11 connection rejected because of wrong authentication.
Error: Can't open display: localhost:10.0
(hlovdal) remotehost:~>ls -l /home/hlovdal/.Xauthority
-rw-------. 1 hlovdal hlovdal 70 Jul 25 23:30 /home/hlovdal/.Xauthority
(hlovdal) remotehost:~>

What might be the cause here?

(This is between my two laptops where I have shh-ed back and forth hundreds of times before.)

Update:

While debugging further, xauth again complained that it could not write to .Xauthority
This does not make sense since the file is writable by my user,
and out of curiously I ran strace to see exactly what it tried to do.

(hlovdal) remotehost:~>xauth list
xauth:  /home/hlovdal/.Xauthority not writable, changes will be ignored
(hlovdal) remotehost:~>strace -oout -f -s9000 xauth list
xauth:  error in locking authority file /home/hlovdal/.Xauthority
(hlovdal) remotehost:~>tail out
16634 brk(0xf03000)                     = 0xf03000
16634 brk(0)                            = 0xf03000
16634 rt_sigaction(SIGINT, {0x4050d0, [INT], SA_RESTORER|SA_RESTART, 0x3957035350}, {SIG_DFL, [], 0}, 8) = 0
16634 rt_sigaction(SIGTERM, {0x4050d0, [TERM], SA_RESTORER|SA_RESTART, 0x3957035350}, {SIG_DFL, [], 0}, 8) = 0
16634 rt_sigaction(SIGHUP, {0x4050d0, [HUP], SA_RESTORER|SA_RESTART, 0x3957035350}, {SIG_DFL, [], 0}, 8) = 0
16634 rt_sigaction(SIGPIPE, {0x4050d0, [PIPE], SA_RESTORER|SA_RESTART, 0x3957035350}, {SIG_DFL, [], 0}, 8) = 0
16634 stat("/home/hlovdal/.Xauthority-c", {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
16634 open("/home/hlovdal/.Xauthority-c", O_WRONLY|O_CREAT|O_EXCL, 0600) = -1 EEXIST (File exists)
16634 write(2, "xauth:  error in locking authority file /home/hlovdal/.Xauthority\n", 66) = 66
16634 exit_group(1)                     = ?
(hlovdal) remotehost:~>ls -l .Xauthority*
-rw-------. 1 hlovdal hlovdal 70 Jul 25 23:30 .Xauthority
-rw-------. 2 hlovdal hlovdal  0 Jul 26 01:26 .Xauthority-c
-rw-------. 2 hlovdal hlovdal  0 Jul 26 01:26 .Xauthority-l
(hlovdal) remotehost:~>

This explains better why xauth fails since it fails to create .Xauthority-c.
But removing those two probably stale files does not help, they are recreated on the next ssh login.
syslog in remotehost contains the following related to the ssh login.

sshd[17551]: Accepted publickey for hlovdal from x.x.x.x port 36545 ssh2
sshd[17552]: fatal: mm_request_receive: read: Connection reset by peer
sshd[17551]: pam_unix(sshd:session): session opened for user hlovdal by (uid=0)

This mm_request_receive error message have never occured before, so I guess there is the clue for further debugging.