Warning: preg_match(): on every page

When I install your plugin in a staging site I get the following error:

Warning: preg_match(): Compilation failed: quantifier does not follow a repeatable item at offset 1 in /…l/wp-includes/class-wp.php on line 238

Have you seen this before? I have reset my permalinks and the error still exists.

The page I need help with: [log in to see the link]

 

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