Intermittent Azure VPN Connection Issue – “Server did not respond properly to VPN control packets” (AAD Auth / SSL TCP)

Hi Singh, Saurabh,

Thanks for reaching out in Microsoft Q&A forum,

Based on the information provided, the intermittent "Server did not respond properly to VPN control packets" error cannot be conclusively linked to a single root cause. While Entra ID (Azure AD) token or credential cache issues remain a possibility, the fact that the issue affects multiple users, devices, and VPN profiles across both Dev and Prod environments suggests that it is unlikely to be caused solely by an expired token or a corrupted local profile.

The observed behavior is more consistent with an intermittent issue occurring during the connection establishment process between the Azure VPN Client and the Azure VPN Gateway. Since the VPN connection succeeds at times and fails at others, the issue may be related to communication interruptions rather than a permanent configuration problem.

Possible contributing factors include:

• Intermittent communication issues between the Azure VPN Client and Azure VPN Gateway.
• Client-side session or authentication cache issues.
• Network devices such as firewalls, proxies, or SSL inspection appliances interfering with VPN traffic.
• Temporary Azure VPN Gateway responsiveness or backend service issues.
• Entra ID authentication flow or Conditional Access policy-related interruptions.

The fact that clearing saved credentials and synchronizing system time temporarily improves the situation may indicate that authentication or session state plays a role. However, this alone is not sufficient to conclude that refresh token expiration is the primary cause, especially given the broad impact across multiple users and environments.

To further isolate the issue, it would be beneficial to review:

• Azure VPN Client logs from affected devices.
• Azure VPN Gateway diagnostic logs.
• Entra ID sign-in logs during the failure window.
• Windows RasClient and Schannel logs for authentication or TLS-related errors.

Official Microsoft documentation:

• Troubleshoot Azure VPN Client: https://learn.microsoft.com/en-us/troubleshoot/azure/vpn-gateway/troubleshoot-azure-vpn-client
• Troubleshoot Point-to-Site Connection Problems: https://learn.microsoft.com/en-us/troubleshoot/azure/vpn-gateway/vpn-gateway-troubleshoot-vpn-point-to-site-connection-problems
• Q&A on This Error: https://learn.microsoft.com/en-us/answers/questions/1658973/server-did-not-respond-properly-to-vpn-control-pac
• Q&A on Token Refresh: https://learn.microsoft.com/en-us/answers/questions/5836928/azure-vpn-connection-disconnects

Kindly let us know if the above helps or you need further assistance on this issue.

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Hi Saurabh, the "Server did not respond properly to VPN control packets" error shown is almost certainly an Entra ID authentication token expiration or clock-skew issue clashing with your SSL TCP transport. Because TCP requires strict packet ordering, any slight delay while checking your AAD token or CRL causes the gateway to drop the handshake entirely. Since clearing caches and resetting NTP only offer temporary relief, your best long-term fixes are switching the VPN protocol to UDP (OpenVPN) to bypass TCP packet-drop sensitivity, checking your Entra Conditional Access sign-in frequency policies for aggressive token expiration limits, or verifying that your Azure VPN Gateway SKU isn't hitting resource limits during user spikes.