Error when updating Plugin

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Hi,

I updated the plugin to the latest version (3.5.1) and then when testing my Contact Form (Contact Form 7) I found that when clicking in “Send message” an error message would appear and the message would not be send.

In the plugin when testing the email this error appears:
Attention! The last email your site tried to send was not sent successfully.
Mailer: Otro SMTP
SMTP Error: Could not authenticate.

I have the same plugin in several sites, but this error appears only in the two sites I updated the plugin.

I already tried installing a previous version, but nothing changes.

What would you think the problem would be?

Thanks in advance.