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I was importing a test version of IPB 4.4.10 to Xenforo 2.2. It started fine and then suddenly I got the above.
Both were protected by .htaccess but my ip and 127.0.0.1 were both allowed.
Server cpu Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
At this moment 64 GB Ram and at least 35 GB ram free.
It happened during importing threads.
I reloaded the page and at this moment it's going further, but I would really like to know what is causing this and how to prevent this on the real import.