It occurs with a jiraProjectPages module, when clicked on in the apps section a project very immediately the error box appears, but the app runs fine after that. It seems, the error happens before the JavaScript of the app is loaded.
I stripped down atlassian-connect.json to one module:

"modules": {
"jiraProjectPages": [
{
"url": "/admin-rules-manager?readOnly=true&projectId={project.id}&projectKey={project.key}",
"iconUrl": "/images/palmtree.png",
"weight": 100,
"name": {
"value": "Out-Of-Office User View"
},
"key": "user-rules-overview"
}
]
},
replaced the app with a Hello World! (which appears)
{{!< layout}} <div id='admin_rules_manager' projectKey='{{{projectKey}}}' projectId='{{{projectId}}}'
slackClientId='{{{slackClientId}}}' isNewSite='{{{isNewSite}}}' readOnly='{{{readOnly}}}'
data-options='sizeToParent:true'>
</div>
<!-- <link rel='stylesheet' href='/js/AdminRulesManager.css' />
<script src='/js/AdminRulesManager.js'></script> -->
Hello World!
still it shows the error. From aui I use:
<script src="//aui-cdn.atlassian.com/aui-adg/9.6.0/js/aui-soy.js" type="text/javascript"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/9.6.0/js/aui.js" type="text/javascript"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/9.6.0/js/aui-datepicker.js"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/9.6.0/js/aui-experimental.js"></script>
What could be the problem?