Internal Server Error on Heroku

[aquilesC]

The deployment to Heroku worked fine, but when I tried to submit the SMTP credentials I got a 500 Internal Server Error. The log says this:

ValueError: Missing staticfiles manifest entry for 'vendor/bootstrap-4.1.3/css/bootstrap.min.css'

This is very strange, because it happens only after I try to update the values, but the template used for both GET and POST requests is the same.

Update:
I get the error only when setting the port to 465, if I use 25 or 587 there are no issues. This points me into the direction of the form verification used to validate data (this). I'l need to debug it further to understand what is happening and why the error is with the staticfiles manifest