While testing some new AbuseFilter features on Beta Cluster, I noticed that something in OOUI 0.31 is causing the error in the title. Specifically, if you head to https://deployment.wikimedia.beta.wmflabs.org/wiki/Special:AbuseFilter/test, you'll see that the form is missing a field ("Changes made after:") and the following ("Changes made before:") isn't fully initialized. Both of these are datetime fields, and this is what's logged in the console:
TypeError: this.setFlags is not a function at MwWidgetsDatetimeDateTimeInputWidget.mw.widgets.datetime.DateTimeInputWidget.updateFieldsFromValue (<anonymous>:135:308) at MwWidgetsDatetimeDateTimeInputWidget.mw.widgets.datetime.DateTimeInputWidget.setupFields (<anonymous>:134:738) at new MwWidgetsDatetimeDateTimeInputWidget (<anonymous>:129:479) at Object.OO.ui.Element.static.unsafeInfuse (load.php?debug=false&lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|oojs-ui.styles.icons-editing-advanced&skin=vector&version=0gwg1ug:160) at deserialize (load.php?debug=false&lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|oojs-ui.styles.icons-editing-advanced&skin=vector&version=0gwg1ug:160) at Object.oo.copy (<anonymous>:76:635) at Object.oo.copy (<anonymous>:77:162) at Object.OO.ui.Element.static.unsafeInfuse (load.php?debug=false&lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|oojs-ui.styles.icons-editing-advanced&skin=vector&version=0gwg1ug:160) at Object.OO.ui.Element.static.infuse (load.php?debug=false&lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|oojs-ui.styles.icons-editing-advanced&skin=vector&version=0gwg1ug:158) at Object.OO.ui.infuse (load.php?debug=false&lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|oojs-ui.styles.icons-editing-advanced&skin=vector&version=0gwg1ug:154)
I tested this on my local wiki with core+OOUI ~2 weeks old and it didn't happen, so I guessed that this comes from OOUI 0.31.