How to debug unresponsive svn server?

Question

The SVN server running on a Synology NAS become unresponsive (after years of operation): "Unable to connect to a repository"

• I can access the NAS through ssh as well. High-level logging is enabled in the svnserve command, but the log is empty.

• I've run a find /var/log -type f -exec grep -nHi svn {} \; but nothing indicates any traffic to the svn repository.

• According to ps -aux the svnserve process is still running (STAT=Ss).

• I've checked with wireshark, that a TCP packet goes out to the port 3690 at server's IP address after an svn up command on the client side.

• There is enough disk space and memory, both on the server and on the client side.

• Reboots does not help neither on server nor client side.

• My client version is 1.14.2. According to the GUI the SVN server is on the newest version: 1.14.0, but I got no answer from svnserve about its version. I find this suspicious:

   $ /var/packages/SVN/target/bin/svnserve --version
   svnserve: E200029: Couldn't perform atomic initialization
   svnserve: E070023: Can't create fsync thread pool in FSX: This function has not been implemented on this platform
  

• I've tried the above command with sudo as well, to check whether it has some file permission problems.

• I've also checked that the root of the repository is at the location, what was used in the svnserve command showed by ps -aux.

• Synology support claims, that it is a third-party software and they can't help.

How could I make sure whether the requests reach the svnserve process, or are filtered out before? What else could I do to narrow down the source of the error?