We can't really tell anything without some data. Run your VI. Make the value in read buffer (hex) default. Remove all the VISA stuff. Change read buffer (hex) to a control. Does the problem still exist? If so, post that version. If not, put some error clusters in your program and report what errors you see.
Also, tell us what values you think should occur on each of the indicators, so we can tell what you think is wrong.
1. I noticed that you are using the "Run Continuously" button in the error image. Does the error occur on the first run or on one of the later ones? The port may not be able to close and reopen as fast as LV can run the VI again. Try using just the Run arrow to let it run one time.
2. What is the exact string received (including any non-printing characters) from the VISA Read the first time it runs?