Have you:

Rebuilt permissions at:
example/admin/content/node-settings

I know this probably isn't it but, in case you missed noticing that viewing comments and posting them are two different permissions...
example.com/admin/user/permissions

Seems like something is weird... have you done stuff at he db level like "repairing database" and "optimizing" tables.

Also, when I run into "weird" stuff, I always empty out ("truncate") all the rows in the "sessions" table in addition to emptying all cache tables.

I'm definitely shooting darts in the dark, maybe someone else can think of better stuff, but...

-- 
Michel 'ZioBudda' Morelli                       michel@ziobudda.net
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