ActiveRecord::ConnectionTimeoutError: could not obtain a database connection within 5.000 seconds (waited 5.000 seconds)

I had the same problems which were caused by too many open connections to the database. This can happen when you have database queries outside of a controller (in a model, mailer, pdf generator, ...).

I could fix it by wrapping those queries in this block which closes the connection automatically.

ActiveRecord::Base.connection_pool.with_connection do
  # your code
end

Since Puma works multi-threaded, the pool size (as eabraham mentioned) can be a limitation, too. Try to increase it (a little)...

I hope this helps!

With the help of the devise guys' I think I finally got this issue figured out. It seemed that by using custom error pages with it's own controller, I wasn't skipping the before_action get_new_messages. So the very simple fix was to add:

skip_before_filter :get_new_messages

to my custom error controller.

This issue explains in detail the reason behind this: https://github.com/plataformatec/devise/issues/3422