Please have look at issue 1.7.32: Why impose max sizes for field types? Admin fails on large content sizes. · Issue #3643 · getgrav/grav · GitHub.
For some reason, Grav 1.7.32 has added a default max size of 65536 bytes for text (multiline) and textarea fields of pages. Admin fails when adding larger content sizes because of this validation, however, Grav itself seems to be functioning fine with larger content sizes.