Hello,
I experiencing the above problems. I found that, neither in TinyMCE Advanced nor in Classic Editor Text tab is working (nothing happens when I click it). Also, as mentioned above Yoast fields are not visible. I tried to disable TinyMCE Advanced, but unfortunately same issue applies for Classic Editor as well. The following error appears in the console (back end, on page/post edit screen):
admin.js?ver=1.0:1 Uncaught ReferenceError: default_global_customization_type is not defined
at HTMLDocument.<anonymous> (admin.js?ver=1.0:1)
at i (load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils&ver=5.1.1:2)
at Object.fireWith [as resolveWith] (load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils&ver=5.1.1:2)
at Function.ready (load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils&ver=5.1.1:2)
at HTMLDocument.K (load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,utils&ver=5.1.1:2)
Only when switched to Gutenberg I’m able to edit HTML.
WP version is 5.1.1. Please advice how to proceed.