Today we have seen this error again. Restart of Gerrit fixed this issue.

I have look a bit and found the following thread:

According to the above, there's a bug in Jgit. Its cache gets corrupted and it cannot find objects.

Restart of Gerrit causes it to clear the cache and that fixes the problem.

In order to fix that, you cannot only upgrade JGit (as it is built in inside Gerrit installation) but you have to upgrade Gerrit. According to Gerrit's release notes, this issue had been fixed only since Gerrit 2.13.7

It is not urgent but looks like a good idea to upgrade to this version or the latest (2.13.8).

Gil