'Access Denied' for admins

Hi all,

I am 100% ignorant of the workings of Drupal and its modules. Our webmaster has disappeared without a trace and we're trying to fix some bugs, most urgent of which is this.

1. Recently migrated to new shared host
2. Imported site, few issues but mostly working
3. Updated all modules and Drupal core to 6.33 (not sure how to get any higher than that)
4. Administrators (including uid 1) are unable to Masquerade - can access the Masquerade menu, hit 'Masquerade as user', then an Access Denied error is returned

This is what comes up in the recent log entries:

Type access denied
Date Monday, September 8, 2014 - 1:07pm
User admin
Location http://mysite.com/masquerade/switch/558
Referrer http://mysite.com/user-businesses
Message masquerade/switch/558
Severity warning

and:

Type access denied
Date Monday, September 8, 2014 - 1:08pm
User admin
Location http://mysite.com/masquerade/switch/sites/all/themes/chessboard/images/l...
Referrer http://mysite.com/masquerade/switch/558
Message masquerade/switch/sites/all/themes/chessboard/images/loadingAnimation.gif
Severity warning

None of the other issues related to this seem to have been solved, but my apologies anyway for opening a fresh one. And apologies once again for having no idea what I'm doing.

Any help is greatly appreciated.

Thank you.