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Is it particular threads that always do it, or is it just for certain periods of time?

If it's particular threads, are they all made by the same person/people? If someone puts you on their ignore list, you can't post in any threads they make - maybe it's an extrapolation of that?

Other ideas:
• Are they threads which you deleted? It's possible that you can't access them because they're in the GAIA MOD only guild thingerjiger where posts go after you delete them but still linked to the main forum because of the delay glitch?
• Post links to threads (if it's just particular threads) so we can see if we can see them? Do you know if your members are experiencing these problems?