In one of my gerrit (version 2.8.5), this issue can be reproduced by upload a new patch to a same change, with just git commit message changed (no file changed compare to previous patch).
error like this:
$ git push sup master:refs/for/master
Counting objects: 1, done.
Writing objects: 100% (1/1), 255 bytes | 0 bytes/s, done.
Total 1 (delta 0), reused 0 (delta 0)
error: unpack failed: error Missing tree c6b72e000e033d6350c4b9400a292cd0842bc950
fatal: Unpack error, check server log
To sup:lib/shapps
! [remote rejected] master -> refs/for/master (n/a (unpacker error))
If I modify some file and amend to this patch,it will be OK to push:
$ git push sup master:refs/for/master
Counting objects: 37, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 1.16 KiB | 0 bytes/s, done.
Total 6 (delta 4), reused 0 (delta 0)
remote: Resolving deltas: 100% (4/4)
remote: Processing changes: updated: 1, refs: 1, done
To sup:lib/shapps
* [new branch] master -> refs/for/master
I remember I did this same action before with no error, this issue seem to come out after upgrade to 2.8.5