Hello,
there seems to be multiple things going on, so we need to address them one by one.
First is with the license, this one is easy. According to your screenshots, you are installing license file for Version 5.2 under the product of Version 5.1. The version of the license is in the 3rd and 4th digit of its serial number, so if you have xx52..., it is a license for Version 5.2.
If your application truly uses Version 5.1, you need to install the licenses for version 5.1. If you do not have them because you have purchased at time when Version 5.2 was released, let me know, and we will send the licenses for Version 5.1 to you, free of charge.
But if your application is already using Version 5.2, you need to go the License Manager again, and in the "Component" drop-down, select the product with the right version (5.2), and then install the license for 5.2 under it.
Next thing is the exception with "The item definition does not exist in the server address space.". This exception always come from the target OPC server, so the ItemID can be fully incorrect (which is not your case), or the OPC server present a dynamic address space where the item does not exist at one moment and then does exist a bit later, for whatever reason. We never "make up" this error. You need to consult this behavior with the server vendor. If this is how the OPC server "should" behave according to the server vendor, then there is this piece of additiona information that can help dealing with it: QuickOPC internally marks items that return this error, and (within one application) does no re-try them for certain period, which defaults to 5 minutes. This is a configuation parameters called TopicRetrialDelay (in milliseconds), so if you want this to happen earlier, you cxan set it to lower number. Whether the items (for subscribed items) should be retried is evaluated, by default, every 1 minute, and is controlled by the TopicRetrialPeriod, so if you set TopicRetrialDelay low, you should set the TopicRetrialPeriod also lower.

Dear Sir,
I apologize for not replying sooner - it appears that we had a technical problem, and some notifications about forum activity have been lost. I will now answer the posts one by one, hopefully during today and tomorrow.

Hello,
in order to be able to analyze this, you first need to look at what data you are receiving in the notifications, especially after they become "problematic". Specifically, analyze the Exception property of the incoming eventArgs: If it is non-null, it contains an exception object describing the problem. What is the error message in the Exception object?
Best regards