Not possible to replace

I try to replace text in my database, I selected all the tables, tried also to select only one that is 5Mb of size, but it always tells me “Er heeft zich een kritische fout voorgedaan op je website. Controleer je postvak-in van je beheer-e-mailadres voor instructies.” If I check my admin email there is no mail and I cannot replace text?How should I solve this.
My PHP memory limit is already to the max 2048Mb.

Thanks for your answers.

Br,
Marc

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