A pity the stack trace doesn't show the exception. There is a fair chance that this problem in org.apache.sshd is fixed in 2.8.0, but I can't be sure from these stack traces.
The daemon gets an exception at the end of KEX, while trying to install new keys.
There was a change for SSHD-966 in 2.5.0, there was a race condition in KEX fixed in 2.8.0, and 2.8.0 also changed the way new keys are installed in KEX.
Would be really interesting to know whether this still occurs with the most recent Gerrit. If so, having such a stack trace and possibly debug logging switched on for org.apache.sshd might help to track this down for good.