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Error Message
MyBB has experienced an internal SQL error and cannot continue.
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you didn’t tell us if you use plugins? it is possible that one of them is guilty if the forum has worked normally in the past.
Without error reports or logging, it’s a guessing game.
In Admin CP > Board Settings > Server and Optimization Options > Error Type Medium,
if it’s currently set to “Hide Errors and Warnings”
set it to “Warnings and Errors” temporarily, then reproduce the error,
it should show you the exact SQL query involved, which usually leads to the culprit.