When I try to open a paragraph rule from a custom portal, logging in directly in this portal, RTE is blank. But when I log with the same user in Dev Studio and than I go to the portal and try to open the same rule, RTE is correctly shown.
Another note, when I flag the option "Source only mode" the section with the code is correctly shown, but when i re-flag it, the screen is stucked on the loading screen.
That's very interesting. If a brand new section with brand new RTE with simple contents show the same issue then it sounds like something common on your app stack that is attributing to this.
I would check for any obvious errors in the logs and console to see if that might help narrow it down.