Re: Unexpected HTTP status 400 'Bad request'.

 Hello​

Is your repository served read-write by other services like svnserve or
eventually through SSH in addition to Apache HTTPS access ?

If so you have to check your repository file permissions: owner, group and
modes (for instance g+w or g+s...)

I guess your repository has been created long ago with a previous version
of Subversion.
What is your repository format version ? Are some revisions packed ? Use
svnadmin info.

Maybe you should use "svnadmin upgrade" to get some new features properly
enabled with Subversion 1.9,
or even use dump/load procedure (or svnsync) to get your repository ready
(and optimized) for Subversion 1.9.

Regards

-- 
Yves Martin