Noted! I missed that. I would suggest checking service user permissions.

If you POST data to an external endpoint, extract multipart/form-data using request.getPartsRefer to sample https://experienceleague.adobe.com/docs/experience-manager-learn/forms/handling-af-form-submissions/submitting-adaptive-forms-to-external-server-article-use.html?lang=en HTH. 

You should explore the Sling scheduler service: https://sling.apache.org/documentation/bundles/scheduler-service-commons-scheduler.html    

Via Document variable type: https://help.adobe.com/en_US/livecycle/10.0/WorkbenchHelp/WS92d06802c76abadb-43a6230012826bec632-7eff.html ?

Follow the kb : How to set the Oak login token session expiration ThanksWasil

Try with parameter /miscadmin?debugClientLibs=true

Boot delegate RSA and BouncyCastle libraries : Installing and configuring AEM 6.3 forms Thanks,Wasil

There have been couple of improvements included in AEM 6.3 with regards to ModelAdapterFactory(6.3.2.1)[2], so please try on latest AEM 6.3 SP + CFP, taking into account the latest Apache Sling Models Implementation[1]Thanks,Wasil[1] Maven Repository: org.apache.sling » org.apache.sling.models.impl ...

Yes, warning in the logs - might be the reason. Analyse the logs carefully.

Increase heap size first. Use correct memory parameters[1] either with jar or <crx-quickstart folder>/bin/start script when starting AEM instance.Check your servlet/custom code for any possible memory leaks.Thanks,Wasil[1] Increase java heap size – /home/edivad