This may be related to a known issue in Tableau Server 2018.1. Is that the version of Tableau Server you are using?
Would it also be possible to look at your Server logs related to this error?
We now have a workaround that may address this issue. An error is occurring in the default timeout value for communications with TabPy. By manually setting this value in the server configuration using Tabadmin we can allow for connections to operate normally. The command to set is <vizqlserver.extsvc.connect_timeout_ms> and the default value to set it to is 1000. If setting this value to 1000 ms causes a timeout error, experiment with extending the timeout time. For more documentation on setting values with Tabadmin refer here:
The exact series of commands in the command line should look like:
cd "C:\Program Files\Tableau\Tableau Server\2018.1\bin"
tabadmin set vizqlserver.extsvc.connect_timeout_ms 1000