I can't reproduce the error. This tracker is also using SMTP to send e-mail, and everything works as expected; same on my dev box. Maybe it's something specific to your environment ?

Additional, debugging information is needed to help understanding the problem. Please follow these steps:

1. temporarily configure your system as follows:

   $g_display_errors = array(
   E_WARNING => 'halt',
   E_NOTICE => 'halt',
   E_USER_ERROR => 'halt',
   E_USER_WARNING => 'halt',
   E_USER_NOTICE => 'halt'
   );
   $g_show_detailed_errors = ON;

WARNING - SECURITY RISK: the 'show_detailed_errors' config can cause MantisBT to display sensitive information about your system. We recommend to restrict its activation to a Test environment, only for as long as necessary. If possible, do not turn it ON globally, instead limit it for specific user(s) using the Manage Configuration page.

2. Reproduce the error

If the output does not help you resolving the issue and you need further assistance,

3. save the output of the error screen, preferably as an HTML page (not a screenshot !)
4. be careful to remove/mask any confidential information if needed
5. upload the results here

Same for me, I was not able to reproduce

Upgrade from 2.16 to 2.21.1

Maybe @Marnix you don't use the official download, but upgrade based on source on Github.
If so, don't forget to run composer install after fetching latest source to upgrade Mantis 3rd party compoenents, e.g. PHPMailer.

Marnix,

You did not provide any feedback; I am therefore resolving this issue as "no change required".

Feel free to reopen the issue at a later time and provide the requested information.