MailPoet not able to send emails after version 3.90

It seems like MailPoet is unable to send out emails with our own local server after version 3.90 or 3.89. I’ve rolled back to version 3.88.2 and was able to send mails with the exact same settings as before.

Under 3.90.1 I receive the following error message: SMTP Error: Could not connect to SMTP host.

The same error message pops when I try to test the send method and send a test mail with MailPoet.

However other plugins still work perfectly fine with the same settings, I run WP Mail SMTP which is still able to send mails with the same settings (only smtp hostname and port, no secure connection, no authentification).