Error "The server sent an improper HTTP response." on commit with Subversion 1.8+ client
Upgrade your Subversion client to the latest version.
Outdated answer:
ON THE CLIENT MACHINE! Open %APPDATA%\Subversion\servers in a text editor and add the line http-bulk-updates = yes, save the file and see if it helps.
If it helps, you'd better configure Apache HTTP Server's httpd.conf with SVNAllowBulkUpdates prefer directive so that all Subversion 1.8 clients could connect without any errors.
If there are more than just you who get this error in your organization and adjusting server's configuration is still unacceptable, you can change the setting http-bulk-updates = yes via Windows Registry so adjusting this on all affected machines can be done via AD Group Policy.
Read more info in Apache Subversion 1.8 Release Notes.
P.S.: faulty network hardware / firewall / antivirus is still the root cause here. The above is just a workaround to revert to the behavior of Subversion 1.7 and older client with neon network library. BTW, I guess that the installed antivirus is NOD32 or BitDefender.
In my case it was problem with nginx's gzip (I run SVNEdge SVN server behind Nginx).
I disabled gzip and everything started working.