One of two things happened if your scan results report QID 150018 Connection Error Occurred During Scan. Either 1) your scan didn’t reach the maximum number of connection errors/timeouts and the scan was able to finish properly, or 2) your scan reached the maximum number of connection errors/timeouts and therefore did not finish the whole scan as expected.
Check the results of QID 150021 and look for this line:
Reached the threshold for the timed-out links: 12
If you don't see this line, the scan successfully finished, but had fewer than 12 connection errors or timeouts. If you do see this line, the scan stopped during the crawl or test phase.
Check QID 150018. On the first line, you should see this message:
Connection lost during [Crawl] or [Test] phase
If the connection failed in both phases, you'll see this line for each phase. Use the last occurrence to determine when the scan actually stopped.
You'll notice inside QID 150018 there's a list of all the URIs and associated response codes that were reported with a connection error/timeout (example: 200 http://www.mywebsite.com/mypage.php). The URIs tells you where the errors happened, and the response codes tells you why. Those links will be ignored in the future to avoid similar problems during the crawl or test phase.
If the scan stopped because it reached the connection error threshold, there's a pretty straightforward workaround:
1) configure a exclude list that includes the links reported in QID 150018,
2) edit the web application and apply the exclude list in the exclusion lists settings, and
3) launch the scan again. If you continue to reach the limit, you will need to repeat these steps until the scan for that web application is finished.