In my workflow file, I defined the SendProcessMailFunction as post function of the initial action

<function type="avalon">
                    <arg name="role">org.ametys.plugins.bpm.workflowsdef.SendProcessMailFunction</arg>  
                    <arg name="users">${allowExperts}</arg>
                    <arg name="subjectKey">plugin.default-bpm-workflow:WORKFLOW_BPM_MAIL_SUBJECT_ACTION_CREATE</arg>
                    <arg name="bodyKey">plugin.default-bpm-workflow:WORKFLOW_BPM_MAIL_BODY_ACTION_CREATE</arg>
                </function>

but I had not still declared the i18n used for subject and body.

As a result, the creation of the process failed. It should not. Catch the error and log an error instead.
Once the creation failed because of SendProcessMailFunction we are redirect to a 500 error page. The process appears in dashboard but when cliking on it there is another 500 error page because of a "workflowId" property not set.