freezing "svn up"

On some Debian/unstable machine with some Subversion working copy,
"svn up" is sometimes freezing. A "strace -p ..." shows that both
the client and the server are waiting on a "read": "read(6," for
the client, "read(0," for the server.

On the client, after almost half an hour, I eventually got the
error:

client_loop: send disconnect: Broken pipe
svn: E210002: Network connection closed unexpectedly

and several dozens of seconds later, the svnserve was killed by
a SIGPIPE (according to strace).

Earlier, it took 17 minutes for "svn up" to complete (instead of
less that one second usually up to a few seconds), without any
error, but the corresponding svnserve was still running for a few
more minutes.

The last one took:
  0.34s user 0.09s system 0% cpu 19:32.19 total

Again, after a few minutes, the svnserve is still running, but
there's still the sshd:

    root │ └─> 16718 sshd: svn [priv]
     svn │ └─> 16725 sshd: svn_at_notty
     svn │ └─> 16766 sh -c svnserve -t -r d_joooj/home/svn/root --tunnel-user=vinc17/cventin
     svn │ └─> 16767 svnserve -t -r d_joooj/home/svn/root --tunnel-user=vinc17/cventin

I haven't noticed any network issue: interactive ssh works fine.
Checking out a new working copy from the same machine was fine
too. But later, a "svn up" on this working copy was freezing
too... well, it took 3:49. But in general, this seems fine from
this second copy, while problems are much more common from the
first one.

Client:

svn, version 1.10.4 (r1850624)
   compiled Jan 23 2019, 03:41:34 on x86_64-pc-linux-gnu

Server:

svnserve, version 1.9.5 (r1770682)
   compiled Jul 30 2019, 02:45:42 on x86_64-pc-linux-gnu

journalctl does not show any issue (except the usual message
"DIGEST-MD5 common mech free" for the svn client).

With another Debian/unstable machine as the client, which is
in the same room as the other one (thus similar conditions),
I haven't noticed any issue.

Any idea?

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)