Redmine fails to start: There was an error while trying to load the gem 'rbpdf'. Gem Load Error is: This installation of RMagick was configured with ImageMagick 6.9.4 but ImageMagick 6.9.5-5 is in use.

After redmine is installed and FreeBSD's package-tree was updated, Redmine did not start (with some nginx error in the browser), and investigating the error log:

less /var/log/nginx/error.log
...
  Message from application: There was an error while trying to load the gem 'rbpdf'.

Gem Load Error is: This installation of RMagick was configured with ImageMagick 6.9.4 but ImageMagick 6.9.5-5 is in use.

Backtrace for gem load error is:

/usr/local/lib/ruby/gems/2.2/gems/activesupport-4.2.7/lib/active_support/dependencies.rb:274:in `require'

/usr/local/lib/ruby/gems/2.2/gems/activesupport-4.2.7/lib/active_support/dependencies.rb:274:in `block in require'

/usr/local/lib/ruby/gems/2.2/gems/activesupport-4.2.7/lib/active_support/dependencies.rb:240:in `load_dependency'

/usr/local/lib/ruby/gems/2.2/gems/activesupport-4.2.7/lib/active_support/dependencies.rb:274:in `require'
...

led to the conclusion that we need to reinstall things here. So tried a remove of the concerned modules, and reinstalled the bundle:

cd /usr/local/www/redmine
gem uninstall rmagick
gem uninstall rpdf
bundle install
service nginx restart

And voila, redmine is back online.