After upgrading SVN on my server, all the post-commit scripts started to fail.
Here’s a fix which worked for me:
Create a file called “servers” with this content in it:
store-plaintext-passwords = no
Place it in a users home directory in a directory called .subversion
If you don’t know the users home directory, try /var/www/.subversion/servers or /var/www/vhosts/website.com/.subversion/servers (for plesk)