"Connection was terminated by host" indicates that the remote server being scanned is closing the connections being sent to it from Acunetix. There are several reasons why this might happen, but typically it is encountered when the server cannot handle the number of requests being sent.
To fix this issue, a smaller number of requests should be sent to the server at the same time. This can be configured as follows:
- In Acunetix, browse to the Target’s Configuration > General tab
- Decrease the Scan Speed as needed
- Save the settings and try again