Using the System Requirements Test what is the value for "max_input_vars"? You can see supplimental information at UBBDev: Issues with PHP max_input_vars Setting

I usually have mine set to a baseline minimum of 5000, the math to figure out a minimum for your environment is in the above article
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There is no "perfect value", and the total is completely relative to your install; the more forums you have the more of a chance you'll have at reaching this limit. To figure out a value for your forums you should take the amount of forums you have (X) and multiply that value by 26 (current fields) and add 4 (additional fields); ex: (X forums * 26 current setting fields)+ 4 additional fields. The result should be the minimum value needed.

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