Sling model exporters thrown MissingExporterException after upgrading to AEM 6.5.6

Hi all,

After upgraded (in-place upgrade) from AEM 6.4.8 to AEM 6.5.6 our custom sling model exporters throw MissingExporterException when invoked with "model" selector.

org.apache.sling.models.impl.ExportServlet.com.myproject.aem.core.models.mobile.LanguageModel Could not get exporter jackson requested by model. org.apache.sling.models.factory.MissingExporterException: No exporter named jackson supports java.lang.String. at org.apache.sling.models.impl.ModelAdapterFactory.exportModel(ModelAdapterFactory.java:1348) [org.apache.sling.models.impl:1.4.10] at org.apache.sling.models.impl.ExportServlet$ResourceAccessor.getExportedString(ExportServlet.java:188) [org.apache.sling.models.impl:1.4.10]

All our custom exporters are using exporter "jackson" and extension "json"

We have around 40 exporters in the project.

All bundles required by this implementation are active

The link to access Sling Models (/system/console/status-slingmodels) is missing from the system console

My questions are:

1. in which cases this exception might be thrown?
2. all our custom exporters extends a super class without a resoucetype, just to extend some common properties. Can this affect the models to not working properly?
3. What should I check to troubleshoot this issue?

Thank you,

Beatrice