Except that message - are there any other issues? Is there some kind of failure related to it or is it just this message?

It does... first two coming top of my head are check if drive is reporting back device (something like mt) and is loaded and/or mounted.

Thanks Hrvoje.
In my case, the message error appeared when I tried to mount a tape or label a tape. I wasn't doing any type of testing or so.

I asked that because it wouldn't be the first time to see error check function is called as something that would worry you when you see it. To explain a bit, assume you do some operation with tape and you have function to test or check status of tape drive. And let's say you call that function read open error. Usually you would print log/error function (to pinpoint module where problem is) and details of problem. If there would be no problem the answer would be something like: The tape drive is ready for use.

So, I'm not saying this is your case, but that was my first association.

It was just that message. I remembered that I got that message once, and I did what I did that time: I reinstall the driver for the tape, disabled the tape and re-enabled. After that, I restar the NetWorker services, and then I was able to work with that device.

Only restarting the Networker services did not fix the problem.