Are you using a custom sql or tableau SQL?
I'd probably check the server first to make sure that it's not being blocked at that end.
I am using custom SQL
I have other workbooks that work fine (data source is a custom sql)
I am having a similar experience in 2018.3.
I am not using custom SQL and I am attempting to create an extract from a PostgreSQL table. The extract begins and then randomly hangs up at various counts. There have been multiple times when the extract seems to have completed however Tableau asks me if I'd like to reconnect to the datasource. Doing so simply connects to the live datasource again. I've been fighting with this for 2 days now.