InvalidArgumentException: Phrase rendering error: Phrase group An unexpected error occurred is not a valid format

I get a bunch of these errors in the log:

InvalidArgumentException: Phrase rendering error: Phrase group An unexpected error occurred is not a valid format • src/XF/Language.php:298
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The underlying cause is line 459 in src/XF/Mvc/Controller.php:
PHP:

$error = \XF::phrase(\XF::phrase('unexpected_error_occurred'));

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