Hello.
In OPC UA terms, a subscription has parameters such as sampling interval, and comprises of one or more monitored items that share the same parameters (and publishing mechanism). So, even with just one subscription, it is possible to subscribe to multiple monitored items, as long as they share the same subscription parameters.
This server indeed supports only one subscription per session, and that is the cause of the problem, because your subscription(s) cannot coexist together with our internal status subscription.
In UaExpert you have probably simply subscribed to multiple nodes on the same subscription in the "Data Access View", which of course works OK. But you can easily see in UaExpert that only one subscription is supported: After subscribing to one node, do Document -> Add, select "Data Access View", and press Add. This will create a second tab, labeled "Data Access View-1". If you now put a node into it, you will immediately get BadTooManySubscriptions.
It should also be noted that the server declares itself (in its ServerProfileArray) as supporting the "EmbeddedUA" profile. But this profile requires that at least 2 subscriptions per session are available. The proper profile for this server should be "MicroEmbeddedDevice".
The question remains, why did not disabling the status subscription help? I suspect you may not be doing it right. Please post the related pieces of code here, and I wil check them.
Best regards