We all know the Client VPN has its weakness and troubleshooting those issues can be somewhat of a pain. To help I am providing a guide of fixes/suggestions found in the community (Not specifically endorsed by Meraki 😉). This is definitely a WIP (Work in Progress) type of guide. I will try to add items and format the guide with time. Ideally in time organizing them by OS/Auth/Error Code is my goal. If you have any suggestions I have missed please reply so I can add them.
GENERIC
Reboot device having issues
Does the MX have a public IP?
Is the client behind a NAT device? Run tracert to verify
Has support changed your encryption settings?
Use a different internet connection (hotspot or mobile) to test
Verify device is not behind the MX trying to VPN to?
Is the MX behind a device doing NAT (ISP Modem – bridge if possible)?
Any other 3rd-Party Software on the affected system that could be causing an issue (Endpoint security, AV, etc.)?
Have you tried a different device to VPN in with? (using a phone helps to rule that it is a device issue and not Meraki )
Uninstalled any existing VPN software?
Good General Info - https://www.ifm.net.nz/cookbooks/meraki-client-vpn.html
WINDOWS
Try using Rasphone.exe (Windows built-in)
Is the device Windows 10 Pro?
Is Xbox Live Networking Service off?
If using Dell machine make sure you remove SmartByte
https://community.meraki.com/t5/Network-Wide/Dell-Laptops-and-VPN-access/m-p/12826#M321
Use known powershell script to install profile
Tried network reset? (netsh int ip reset)
Checked Windows Reliability Monitor for recent updates/issues/performance?
Tried limited boot with majority of services/programs disabled?
Verify you have encryption set to optional, not required on the adapter properties?
MAC
MacOS remove Local Profile -System Preferences -> Networking -> Location profile
RADIUS
Modify msRADIUSServiceType
https://community.meraki.com/t5/Security-SD-WAN/Meraki-VPN-Some-users-get-691-error-when-authenticat...
Try basic preshared key (password)
Change to Meraki Auth if possible to verify
Certificate expired on server?
Verify LAN IP of domain controller hasn't changed
Verify AD domain admin credentials
MERAKI
Verify Meraki credential on the web