StarWind Software

I am using Starwind V5.7 free edition to evlulate Starwind iSCSI software. I created a ibv target to test the snapshot CDP. I checked the server log and found three errors occur in each snapshot. The errors are below:

9/21 1:06:02.484 7b0 func: SnapShot Registry function 'SSRegGetSnapShotIDbyFilename' returned an error code: 11
9/21 1:06:04.218 7b0 func: SnapShot Registry function 'SSRegGetSnapShotIDbyMapFilename' returned an error code: 11
9/21 1:06:04.218 7b0 func: SnapShot Registry function 'SSRegGetSnapShotIDbySessionID' returned an error code: 25

I want to know what the errors mean? How can I fix it?

I setup the maximum snapshot limit to 8 but I found that a number of .ibvd files exist in the folder is more that 8 (actually I see all .ibvd files are kept in the folder). It consumes a lot of space. I want to know why the number .ibvd files is more the the snapshot limit that I specified in the target setup? Is it possible, and how, to safely remove the .ibvd files?

First of all the messages that you`ve posted are not error messages actually, you can see them while aby snapshot creation process. This is just informational messages (we will change their format in future releases).
About your issue: actually it can be done only by adding additional space.
Snapshot is using some of the previous *.ibvd files as the source of the data. ibvd files will be deleted if there will be no data that is required for functioning of new snapshot (btw yes, only new data is writting to the new snapshots).
Also you can create linked clone that and there will be no need to store ibvd files, but that requires space too.