Hello.
First, I have to apologize - the information given earlier was not fully correct, as I have realized by looking at your settings. The .HoldPeriods.TopicRead and .HoldPeriods.ToipcWrite of course apply to reads/writes (I have incorrectly stated that HoldPeriods only have to do with explicit subscriptions).
There is nothing wrong with your settings below. Of course when reading always from cache, one needs to be prepared that the cache is not always filled in with good quality data - especially in the beginning. The "operation failure" information is not sufficient to determine what's wrong. For example, if you have used ReadItemValue/ReadMultipleItemValues (as opposed to ReadItem/ReadMultipleItemValues), the reason may simply be a Bad quality. Or, it can be something else - it is necessary to examing other fields of the exception, especially the InnerException and the Source. Can you please find this out?

Hello,

80 or 800 tags is not high number per se, we work with thousands normally, but it all depends on how often you read/write them, or how often they change (in case of subscriptions).
When you do SubscribeItem, you will get notification all the time until you call UnsubscribeItems. The HoldPeriod is only an internal optimization, and only applies AFTER you unsubscribe. It means (roughly) that after UnsubscribeItem, the actual OPC subscription is not immediately removed, but is held for certain period. The reason why HoldPeriod exists is that many applications such as HMI-like apps have forms that subscribe to a set of items, and if you switch to a different form, there may be many same items, and it would be unnecessary to unsubscribe from an item and then subscribe to it again in just a few milliseconds. If your application is not alike this scenario, you can set HoldPeriod to zero. I do no see, however, how HoldPeriod can have anything to do with your issue.
I'd like to reinstate the fact the "Device failure" quality is something that should indicate device failures, and can only come from the target OPC server. If it is generated in situations when it should not, you need to investigate on the OPC Server side, not on the client side.

Best regards

Hello,
the HoldPeriods are only used with subscriptions that you explicitly make using SubscribeItem/SubscribeMultipleItems.
If you do not use SubscribeItem/SubscribeMultipleItems, and you want to prevent any OPC subscriptions being used internally, you can enforce that by setting both UpdateRates.ReadAutomatic and WriteAutomatic to Timeout.Infinite (-1). UpdateRates.WriteAutomatic is already equal to Timeout.Infinite by default, so in reality you just need to set the ReadAutomatic.
I am not completely sure where you are getting to, so if I have not answered your question in full, let me know.