Solution

Each time after changing the LinkCheckerTask configuration with proxy settings

1. Stop CQ5.
2. Edit crx-quickstart/launchpad/config/com/day/cq/rewriter/linkchecker/impl/LinkCheckerTask.config:
   1. Remove the line setting containing connection.timeout (see also related document about it).
   2. Remove the line setting containing proxy.ntlm.domain.
   3. Remove the line setting containing proxy.ntlm.host.
3. Restart the CQ5 instance.
4. Verify that the LinkCheckerTask component is active.

Additional information

Once the user/password are set for the proxy, it always uses NTLM authentication scheme as the sling config contains empty values for ntlm domain and ntlm host.

Linkchecker is using Apache HttpClient libraries, and thus doesn't support NTLM v2.

Applies to: CQ 5.2 and 5.2.1.