When setting the Instance options of the Virtual Agent Service Portal widget as documented here the Topic does not load after starting a new conversation and reloading the page, and instead you get a message about "An unrecoverable error has occurred." and are transferred to an agent.
The sys_id you entered is most likely from the sys_cb_topic table which is where topics are stored when developing in the designer, and not the sys_cs_topic table which is where topics are stored as runtime data and is the correct place to get the sys_id for the widget options.
When getting the sys_id for your topic to set as the default in the instance options follow these steps.
1) Navigate to sys_cs_topic.list
2) Find the record for the Topic you want to be the default in that widget instance
3) Right-click the record and select copy sys_id.
4) Use that sys_id in the widget options as "sysparm_topic=sys_id" and it should correctly start the widget with that topic as default.
Full Docs link from above: