I am a little late in reporting this, but I hadn’t realized the problem until I had to maintain an older project that used Log4Net 1.2.8. I upgraded it to log4net 1.2.10 and noticed it stopped working. I then found this comment in the log4net mailing list archives.
There were a number of breaking changes in 1.2.9
http://logging.apache.org/log4net/release/release-notes.html#1.2.9
In your config file "log4net.spi.LevelEvaluator" needs to be updated to "log4net.Core.LevelEvaluator".
I hope changes that would break existing config files are far and few between.