Remove log4net.dll in MediaPortal.Common from git repository and reference it using a NuGet package.

Homepages:
http://logging.apache.org/log4net/

Latest available version is 1.2.11.
They've released two different builds: one using the old key and one using a new key. The new key will be used for new releases in future.
More infos about it: http://logging.apache.org/log4net/release/faq.html#two-snks

Current version in MP2 repository is: 1.2.11 (old key)