I am having this exact problem with any form submissions that send emails as of yesterday. Like you I’m experiencing this with internal form submission, like adding a user, or contact forms. Again, anything that emails. So users stared at a blank screen, and were getting a 504 error until we created a temporary workaround by editing the htaccess file which lengthened the timeout period. This isn’t a good long term solution obviously. I timed up to a minute wait for the submissions to go through, and of course users will probably not wait that long or believe that it didn’t work.
I think something happened yesterday that is causing this.. perhaps Gmail updated something that broke the plugin?
