Hi David. What is the actual error and stack trace being logged when you get the Internal Server Error? That will be critical to resolving the conflict. This error will be in the web server error log, or in an email sent from WordPress to the website admin email address. Please let us know what you find. Thank you.
I am getting the same issue as well. When trying to use the oxygen builder the error message I am getting is
Fatal error: Unknown: Cannout use output buffering in output buffering display handlers in Unknown on line 0.
When I disable gravity forms then oxygen builder works and when it is enabled it doesnt. The website has several forms that need to be used and the website has to be updated periodically, the business is in its busiest season and I am trying to get this resolved as soon as possible.