key_verify failed for server_host_key, fatal: Could not read from remote repository.

recently on fisheye.dev and atlaseye we are getting few dozens errors each day when trying to hit stash to do incremental indexing. Those relates to different repositories, e.g.:

[2014-06-26 07:52:53,658] WARN  fisheye
Exception during FishEye Incremental Indexing of BLTZ-adam2: com.cenqua.fisheye.config.ConfigException: Unable to fetch from remote repository: ssh://git@stash.dev.internal.atlassian.com:7999/bltz/adam2.git
- [key_verify failed for server_host_key, fatal: Could not read from remote repository., , Please make sure you have the correct access rights, and the repository exists.]

and seems to be intermittent. When I go to fisheye.dev now and check BLTZ-adam2 repository, Test Connection is successful and have no issues connecting to it.

Any ideas what could be the cause? Don't see such errors for bitbucket repositories. Is this possible that when stash is in the maintenance mode (e.g. upgrade) nginx reverse proxy replies for example with different ssl cert?