After some cookie-cutter cut'n'paste a while ago, the svn paths in my Bamboo Plans have a number of variables in the middle of them, in this case '$bamboo.name = "project"'. The automatic branch detection code doesn't work on this, throwing errors in the log like:

[BambooPluginUtils] An unexpected error has ocurred while detecting branches
Caused by: com.atlassian.bamboo.repository.RepositoryException: Path
https://subversion.example.com/svn/$%7Bbamboo.name%7D/branches does not
exist. Please check configuration of SVN repository in this job.

There is some replacement of Bamboo variables occurring, as the page describing where Bamboo is going to look for branches (Configuration -> Branches -> Subversion Branches URL) fills out the URL as:

https://subversion.example.com/svn/project/branches

and obviously Bamboo is able to replace the variables and check out the subversion path for the purposes of building the project.

Please pass all instances of the subversion path through the Bamboo variable reconfigulator machine before using them.

(And your Jira instance no longer knows about version 4.3.1 of Bamboo)