404 settings not saved

[eluser]Unknown[/eluser]
I have seen this issue posted before, but no solution.
I am running EE 2.7.3.
When I trie to set the 404 page on Global Template preferences and click on save, nothing happens. I.e. the setting is not saved.
Non existing pages show no error message. The page remains blank.

Removing the htaccess does give the standard browser error.
The same htaccess in other sites does not prevent the custom 404 page showing, so that doesn't seem to be the cause.

Any thoughts?

[eluser]CroNiX[/eluser]
You might get more help if you post this in one of the EE specific forums.