Error while trying to email an invoice (The remote name could not be resolved: 'my.manager.io')

When I try to email an invoice, I get the following error: The remote name could not be resolved: 'my.manager.io
I have emailed an invoice before (a few weeks ago), so the settings should all be correct.
I do not use a custom SMTP-server.

What should I do to fix this?

Make sure your software is updated. There were some recent changes that fixed this problem for others. Let us know if that works.

That was the problem.
Thank you for the fast reply!