Strange VPN Issue - RDP & ICMP

[AIRCo_Andrew]

Since 12/5, we have a window each morning where RDP & ICMP traffic completely drops. It is probably more types of traffic, but those are the two protocols we've observed and been able to replicate. Users are disconnected from RDP, but the VPN stays up. The window typically occurs anytime between 7:30-9:30am and usually lasts around 30 minutes. The remainder of the day sees no issues at all.

Things I know/have done/eliminated/etc...:
No known network changes
No known internal processes or anything new that is impacting network performance

No known commonality between the users and servers, other than the users being on the VPN and using RDP.
Nothing in Event Logs or Security Center

Contacted Meraki Support, but they don't see anything on the backend or anything that stands out
Hardware utilization is fine
Pings/RDP from internal servers to other internal servers and external destinations are fine 

I'm really not sure where to go from here. Anyone ever experienced this?

[RWelch]

What firmware are you running?

[AIRCo_Andrew]

We have a MX450 with MX 18.211.4 firmware

[RWelch]

Windows Firewall tends to be the most common culprit for RDP issues.  
MX 18.211.4 has been the most stable release in my networks/organizations.  

[AIRCo_Andrew]

I would normally agree but nothing has changed with that either. Either way I'll look into that. Just such a weird issue.

[PhilipDAth]

Have you checked circuit utilisation?  Perhaps it is just getting maxed out.

Does other types of traffic, like ping or SMB continue to work?

What do your uplink statistics show?  Any growing packet loss?