Via cluster user report. Here's at trace from web.
[2020-04-27 20:48:38] EXCEPTION: (RuntimeException) Undefined index: at [<arcanist>/src/error/PhutilErrorHandler.php:253] arcanist(head=stable, ref.master=7d15b85a1bc0, ref.stable=c87880142b72), libcore(), phabricator(head=stable, ref.stable=a1c978ef378a), services(head=stable, ref.master=fb2140e5b9da, ref.stable=2d7586076ae4) #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/differential/parser/DifferentialChangesetParser.php:984] #1 <#2> DifferentialChangesetParser::render(NULL, NULL, array) called at [<phabricator>/src/applications/differential/parser/DifferentialChangesetParser.php:75] #2 <#2> DifferentialChangesetParser::renderChangeset() called at [<phabricator>/src/applications/differential/parser/DifferentialChangesetParser.php:1874] #3 <#2> DifferentialChangesetParser::newChangesetResponse() called at [<phabricator>/src/applications/differential/view/DifferentialChangesetListView.php:207] #4 <#2> phutil_escape_html(DifferentialChangesetListView) called at [<phabricator>/src/infrastructure/markup/render.php:135] #5 <#2> phutil_escape_html(array) called at [<phabricator>/src/infrastructure/markup/render.php:97] #6 <#2> phutil_tag(string, array, array) called at [<phabricator>/src/view/phui/PHUITwoColumnView.php:243] #7 <#2> PHUITwoColumnView::buildFooter() called at [<phabricator>/src/view/phui/PHUITwoColumnView.php:123] #8 <#2> PHUITwoColumnView::getTagContent() called at [<phabricator>/src/view/AphrontTagView.php:161] #9 <#2> AphrontTagView::render() called at [<phabricator>/src/view/AphrontView.php:222] #10 <#2> AphrontView::producePhutilSafeHTML() called at [<phabricator>/src/infrastructure/markup/render.php:111] #11 <#2> phutil_escape_html(PHUITwoColumnView) called at [<phabricator>/src/infrastructure/markup/render.php:167] #12 <#2> phutil_implode_html(string, array) called at [<phabricator>/src/view/page/PhabricatorBarePageView.php:58] #13 <#2> PhabricatorBarePageView::willRenderPage() called at [<phabricator>/src/view/page/PhabricatorStandardPageView.php:216] #14 <#2> PhabricatorStandardPageView::willRenderPage() called at [<phabricator>/src/view/page/AphrontPageView.php:46] #15 <#2> AphrontPageView::render() called at [<phabricator>/src/view/page/PhabricatorStandardPageView.php:904] #16 <#2> PhabricatorStandardPageView::produceAphrontResponse() called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:723] #17 <#2> AphrontApplicationConfiguration::produceResponse(AphrontRequest, PhabricatorStandardPageView) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:303] #18 phlog(RuntimeException) called at [<phabricator>/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php:41] #19 PhabricatorDefaultRequestExceptionHandler::handleRequestThrowable(AphrontRequest, RuntimeException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:752] #20 AphrontApplicationConfiguration::handleThrowable(RuntimeException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:341] #21 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:211] #22 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:35]