Steps to reproduce:

1. Login to back office.
2. Go to Content / Content structure.
3. Click on 'Search' button at the left side menu.
4. Click on 'Filters' button.
5. Right after filters menu expand, start typing 'Administrator user' in input field 'Creator' and then select 'Administrator user'.
6. Click on 'Apply' button.

Actual result:

See attachment.

Watch gif - link: https://recordit.co/jFzUufnFgI

An error code 500 is throw while searching for a content using filter by creator.

Expected result:

User should be able to search for a content without any errors.