Just had the same symptoms because of incompatible DNS resolver plugin - it can be recognized by specific errors in logfiles.
Check this thread, may be it helps: Rolling upgrade from 2.3 to 2.4?
Just had the same symptoms because of incompatible DNS resolver plugin - it can be recognized by specific errors in logfiles.
Check this thread, may be it helps: Rolling upgrade from 2.3 to 2.4?