RE: you do not you do not have permission to access this page – … while I’m an administrator. If I go to manage an entry and then return to categories, it is unlocked
Please try deactivating W3TC Cache. Try WP Rocket instead if you want page caching for your site. It has worked much better for me. Just my personal experience.
Also, I see you have two SEO plugins installed. Perhaps you should only use one?
Lastly I see you have two security plugins installed. I recommend choosing one. These can cause increased page load times as they have to validate each request.
I do not think any of these could cause the issue you are experiencing. But trimming plugins in use is a good start in narrowing down the cause.
RE: Date picker
The theme is adding a lot of padding to all dropdowns globally.
Add I added this to the theme’s custom CSS area in the Customizer:
.ui-datepicker .ui-datepicker-title select {
padding: 0;
}
This corrects the date picker so it displays like it should.
Hope this helps!