500 Internal Server Error when creating issue simultaneously from a number of hosts

We are creating issues in GitLab by using the API version 3 and that usually works pretty well. But if parallel tasks on different hosts are creating issues at the same time, one or some of those requests get a 500 Internal Server Error. We were not able to find anything in the logs, but that may be due to the fact that we don't know where to look. We're running a self-hosted GitLab instance at version 8.13.4

So the questions are: is it possible that parallel processes are causing such an issue and how can we find out what exactly goes wrong?