The push error came up when a previously tried push was unsuccessful with message “Communication breakdown with stash“ and then was successfully retried again due to a supposed network issue. From BB server log: (anonymized)
2018-08-27 03:47:20,494 WARN [threadpool:thread-1] nnnnnn @F9X1A9x227x14151730x4 nnnnn xx.xxx.x.xxx SSH - git-receive-pack '/xxxxx/xxxxxxxx.git' com.bitbooster.hook.MyMergeCheckHook Control Freak - pre-receive failed: com.atlassian.bitbucket.ServerException: An error occurred while executing an external process: Java heap space
com.atlassian.bitbucket.ServerException: An error occurred while executing an external process: Java heap space at com.atlassian.bitbucket.scm.git.command.GitCommandExitHandler.evaluateThrowable(GitCommandExitHandler.java:113) ~[na:na] CRASH of CF!
What helped: Create branch on Bitbucket, and push the changes again to this branch, the push operation is successful (and no crash)