I’m happy to give you the key, yet I did a couple of tests that suggest perhaps the key is not necessary.
This is very strange. It appears to be local to my main site.
In my own testing, I went to a sub-site that also has the Slider Revolution Add-On and is running X (not X Pro). The post worked fine, with Gutenberg (https://peterfae.intothemythica.com/2019/04/02/test-gutenberg/)
This suggested that the network plugins were not an issue (and I had tried to fix it earlier w/plugin conflict tests)
So I activated the plugins on the main site again, including Related Posts Slider, and turned it to 2019.
It worked. The post could be seen and the related posts slider was there.
Then I shifted to the X theme. It didn’t work. Then I tried the X Child, X Pro and the normal X Pro Child themes. Again, same issue. There was apparently something in the theme, on the main site.
So then I considered what else could it be. Could it be something on the SFTP level? Some adjustment to the code? The clean versions of X … perhaps it’s a customization thing?
So I cleared out all the CSS, the Jquery, ‘additional CSS’ and the like. Still nothing.
Again, switched back to 2019, with normal plug-ins and CSS and what-not activated.
Gutenberg worked with Related Posts slider.
Shifted back to X Pro Child. Did not work.
So strange, because it works fine on the subsite which is also using X and Related Posts slider. Do you need the key?
I can give you access