OAuth 2.0 Bearer tokens do not work with addons that declare an ADMIN scope. When an addon attempts to call an API with a bearer token, JIRA / Confluence returns a 500 error.
At the moment we have no workaround for the issue. We have identified the cause and are working to get the fixed release out as soon as possible. Note that the issue affects instances with Atlassian Connect 1.2.1 installed.