Validation error on settings page

[Joerit]

short description: We're updating our redmine to version 4.x. After updating and moving all of our plugins over, we can't update the settings for this plugin anymore via the actual settings page, because it keeps throwing the "Detected non-unique repository identifiers. Cannot switch to flat mode" error

We're not even trying to switch to flat mode. Hierarchical mode is already set, and it's not being changed.

digging through the code, it looks like a simple validation error in /app/forms/plugin_settings_validation/redmine_config.rb.

[Joerit]

I forked and put in a pull request (#745)

Pretty new at this so please, tell me if you need anything more :)