Application tunnel Failed to download configuration

Hello,

i am facing a very wired issue, i have configured APM portal access with application tunnel and it was working perfectly for long time but suddenly application tunnel fail to connect and give error: Failed to download configuration.

any idea to solve this issue note that:-

• i upgraded device version to 16.0.1.1 from about month and application tunnel was working after upgrade.
• application tunnel agent version is 7185 which is compatible with device version.
• application is accessible through portal access but application tunnel fails ( no network connectivity issue).
• changes made on APM policy since upgrade is changing Radius server address and i dont think its the issue and activate IP Intelligence.

please if there is any idea or hint to troubleshoot this issue, our remote users cannot work because we replaced VPN with APM portal access