AppMon: all versions with classic agents
A .NET agent is not delivering any PurePaths and in the agent overview the state "
Fix network connectivity problems
Usually network connectivity issues are causing this problem, so this message comes with connectivity errors in the agent log. In case the collector and/or the monitored application hosts are running virtualized on vmWare, likely it is a known problem with high packet loss.
Exclude huge assemblies from instrumentation
If this does not apply or the NIC buffers are already at maximum, this could also be caused by very high processing times on collector side because of big assembly file size, containing huge amounts of classes, or due to special obfuscation. Known are already certain 3rd party assemblies from vendors like Aspose or DevExpress, which can cause an in general non-critical disconnect, but in case immediately afterwards one essential assembly is tried to be instrumented, the disconnected state will likely trigger the same problem. Those assemblies can - like other non-essential assemblies - be excluded from instrumentation following this KB article: How to exclude assemblies from instrumentation
Switch to AppMon agent
The new AppMon agent platform is performing instrumentation on the agent side, so is not affected by this kind of issues. Please refer to the agent platform introduction and the instructions how to switch.
With the classic AppMon agents, the instrumentation is performed on the collector side, so connectivity must be reliable. Additionally some agent functionality is injected into fundamental assemblies every .NET CLR must contain (like mscorlib.dll and multiple System.*.dll assemblies), so processing those is a requirement for the agent to work.