Tired of logging out or being thrown to a 404 page

When I create a form or make changes, if I enter an invalid string in the form, I get logged out or redirected to a 404 page.

For example, I replaced the string: [textarea your-message placeholder “Tell us about your experience here and any suggestions you have for us”] with the string [textarea your-message size:50 placeholder “Tell us about your experience here and any suggestions you have for us”] , then I get thrown to the 404 page when I click “Save”.

How can I figure out what the problem is? Where can I find the list of errors? Why does it redirect to the 404 page instead of displaying the error on the same screen? Also, sometimes it just logs me out and that’s it. And I have to log in again. What should I do? Any ideas?