Just encountered an issue with Fluent SMPT not sending emails and throwing some errors in the browser console. After disabling a few plugins it turned out to be the Collaboration plugin. Please, see attached screenshot below.
I am using Oxygen 4 and WP 6.0 if that helps.
Unfortunately, I cannot give you access to the website for easier investigation. Still, I will set up a test environment and try to replicate the problem on another domain if required.