Something is breaking in Tableau Server when VizAlerts is trying to get the PDF for the viz. Unfortunately, Tableau Server doesn't communicate what the root cause is in the error it provides, so you will need to look through your Tableau Sever logs to determine what the issue is. Find the latest instance of this happening, then look through your apache access logs for the request that resulted in the 500 code. From there, grab the request ID, and the next step will be looking through the vizql logs for that request ID. Typically you'll see something in there that will help you figure out what occurred.
One thing that might help is to look at the times that this occurs. Is there a pattern? Does it happen at certain times of the day? Does the data source it connect to undergo any regular maintenance that might explain the issue?
If you can't figure it out, you might send a case to Support. They don't support VizAlerts, but a 500 isn't an issue with VizAlerts, it's a failure in Tableau Server, which should be within their scope.