__Steps to Reproduce__
.form file
The error is logged because the check whether the file is a BPMN file tries to parse the JSON as XML.
__Expected Behavior__
The check whether the file is a BPMN file returns false without an error logged.
__Environment__
Can you share an example file? For me opening existing .form files work as expected.

This error seems to occur in conjunction with the Camunda Cloud Connect plug-in. From the stack trace it looks like the plug-in tries to retrieve the relation ID but fails opening forms.
@nikku You're right. I was too quick to assume it is caused by the check whether it is a BPMN diagram. I will this issue and create one in the cloud connect plugin's repository.
Closed in favor of https://github.com/camunda/cloud-connect-modeler-plugin/issues/38.