Unable to set template debugging option...

I’m running an Expression Engine 2.9.2 site.

I’ve just migrated a site from one machine to another to do some development work on it.

The original site works fine, and I can toggle template debugging with no problem.

On the migrated site, while site and CP appear to be fully functional, I am unable to turn on template debugging.

When I try and change the template debugging setting on the CP (in Administration / Output and Debugging Preferences), when I hit submit the CP reports that the preference has been changed, but the setting at the same time reverts to “no” status.

I can change the Display Output Profiler setting above it successfully with no problems: across the whole of the CP so far this is the only element I have found that I cannot change.

I have looked in the config.php file, and can edit the file directly and changes made there are reflected in the CP (e.g. setting cookie_prefix can be done in either CP or in config.php and changes in each place are reflected in the other). However, when I create an entry for template_debugging either via external edit of config.php or via config editor within CP, this value is ignored by the CP.

So I’m baffled. Since the same site works OK in an different instance, I strongly suspect that this something odd like a permissions issue, but I am not able to work out what it is yet.

Any help / suggestions / ideas would be most gratefully received!