Can't edit posts after installation of CPT-Onomies

Have you tried disabling other plugins and switching themes? It’s possible that the problem is elsewhere, as CPT-Onomies doesn’t break the post editor on a WordPress site with default setup (default theme and plugins).

I would try switching to the Twenty Sixteen theme, and if it still doesn’t work, disable your other plugins (except for CPT-Onomies) one at a time until it is fixed. Then you know which plugin is causing a conflict. If you do find one that in conflicts with, please post the name here and we can look into the conflict to find out if the problem is with CPT-Onomies or the other plugin.