When trying to edit commit log message using Tortoise SVN, following error is displayed:

Subversion reported an error:
Repository has not been enabled to accept revision propchanges;
ask the administrator to create a pre-revprop-change hook

pre-revprop-change hook may sound scary, but solution is very simple:

• Login to your Synology NAS web GUI
• Open File Station and navigate to svn > [repository] > hooks
• Find file called pre-revprop-change.tmpl and rename it to pre-revprop-change
  

This is it! Tortoise SVN should now be able to edit log messages.

Some sources say that you also have to make pre-revprop-change file executable, but in my case I didn’t have to do it.

Synology NAS SVN Server 1.9
Tortoise SVN 1.9.3

January 2016