AEM 6.3.3.8 Unable to save content fragment model

I am playing around with content fragments in my local AEM 6.3.3.8 instance. After going through Tools -> Assets -> Content Fragment Models I am able to add a test content fragment model. After I open the model I am able to drag data types (eg single line text) into the model canvas area. After adding a required property name field when I click Save button, it does nothing. I am seeing the following error in the error.log file :

14.04.2020 10:45:50.312 *WARN* [0:0:0:0:0:0:0:1 [1586843150275] GET /mnt/overlay/dam/cfm/models/editor/content/editor.html/conf/sandbox2site/settings/dam/cfm/models/model1 HTTP/1.1] org.apache.sling.xss.impl.XSSAPIImpl Invalid URI.
java.net.URISyntaxException: Illegal character in path at index 117: /mnt/overlay/dam/cfm/models/editor/content/editor/jcr:content/body/items/wizard/items/editorStep/items/content/items/{{field_id}}

There is a long stack trace after these two lines.

Anyone has any suggestion on how to resolve this?

Thanks,

SKM