Symptoms

'Preview Article with Blocks' link is missing

No 'Preview Article with Blocks' for certain users

Cannot preview knowledge article with blocks

Cause

The user's language preference setting was set to 'NUL' which is invalid

Invalid language preference setting in the sys_user table

Check the sys_user record of the user having the issue. Check the language settings (Language/preferred_language) if it is valid.

Change it to a valid language and try again