It's possible to export more than the Maximum number of simultaneous requests defined

This option defined in the global settings should control how many xporter processes can be run simultaneously.

If the User has defined just one at the maximum number on the global settings, the user shouldn't be able to do more than one xporter process simultaneously.

In order to reproduce this error, please, have the attention you should have a large number of issues at your Jira instance in order to give you time to start another process.