While scanning I was getting tons of errors like:

java.lang.NoClassDefFoundError: org/apache/http/impl/client/HttpClientBuilder

Looking into this:

Deadlink depends on httpclient 4.3. In my install this is being omitted due to conflict with httpclient 4.2.1, which is a dependency of the groovy module, and (indirectly) of the rest-integration module...

In my tests adding an <exclusion> where I include the groovy dependency works, httpclient 4.3 is then pulled in and link checking works. I still need to test the groovy module to make sure it runs fine with 4.3, I'll give feedback how that works out. But maybe magnolia could upgrade to httpclient 4.3, or you could downgrade to 4.2.1, otherwise everyone installing either the groovy or the rest-integration modules alongside the deadlink module will have the same problems.