I downloaded the free 10 user version with the plan to migrate from our Jira 10 user version. After getting everything set up I managed to get a project imported ... but only after removing a couple of the Jira users so there were eight left. I believe this was required as there are already 2 users in YouTrack, and if it sees there are more than 10 when importing the Jira users it doesn't import any issues. It creates the project but doesn't import the issues.
This seemed to be the situation even if you have importing of users turned off.
Now I'm trying to import a second project and it's doing the same thing i.e. create the project but not any issues. Even though the import users is turned off I believe it's trying to add users to the already existing 10. No users have been added to Jira ... it still has the same eight as before.