The Logger implementation of log4j-1.2-api casts to org.apache.logging.log4j.core.LoggerContext. This works only if the provider of log4j(2) is the default ("core") provider. I have written my own provider that includes its own implementation of a LoggerContext (properly implementing "org.apache.logging.log4j.spi.LoggerContext", of course). So the cast fails.
IMHO log4j-1.2-api should work with any implementation of a log4j(2) provider.