Please note with anaplan v2 connection in IICS, for anaplan targets only insert operation is supported. Please find the connector doc screenshot below: Thats the reason you are seeing "Operation type 'Upsert' not supported." error when trying to perform upsert.
For the issue where you are seeing Internal DTM error, can you please share the entries that of the error stack that you see in session log. If the session log doesn't have any entries other than "Internal error. The DTM process terminated unexpectedly. Contact Informatica Global Customer Support." please retrigger the task and share the latest entries that you see in tomcat log for analysis, tomcat log path: Informatica Cloud Secure Agent\apps\Data_Integration_Server\logs\tomcat