I can confirm that I also have this issue (2.0.0p12). And I can confirm that it’s from a long running check.
In more detail, it’s from a custom plugin that I’ve written to check the status of our MySQL backups. Every hour it will read the full contents of the backup to verify if the backup was succesful. Now, depending on the size of the databases this can take some time; more than the check interval of 1 minute (that’s why I only run it once every hour).
But I can see that indeed, every hour I now get 1 failed check_mk@blah.service; which is nicely in line with the long run every hour.
For now I’ve set MaxConnectionsPerSource to unlimited and I’ll clean up the failed services every now and then, but this is quite annoying.