Failed to get 2XA file

Symptoms

HTML5 portal fails to detect installed Parallels Client or to launch published applications with it.
There are two or more gateways behind 3-rd party load balancer.

When you enable verbose logging for gateways, C:\ProgramData\Parallels\RASLogs\HTML5GW.log on the servers contain records like these:
[D 37/00000000] Tue Oct 01 16:15:36 2019 - Status: 403  /native/get2xa/8WbbjKeVhTw0VsvTmIOoYqtXOLOupFgz&
[D 37/00000000] Tue Oct 01 16:22:41 2019 - Status: 403  /native/get2xa/ATfxATKE87MwZUvFUpMa3nC2qIE1GGl2&
[D 37/00000000] Tue Oct 01 16:23:42 2019 - Status: 403  /native/get2xa/yDluXkWWdBFPGGeVmcptowmLawjTMw7u&
[D 37/00000000] Tue Oct 01 16:24:09 2019 - Status: 403  /native/get2xa/GRD8SgRVDJQWqMwv93gic5QQ1rb9nOst&

These records correspond to errors in %APPDATA%\2XClient\ClientLog.txt of the affected user:
[E 02/00000037/T6AFC/P67E4/S0001] 01-10-19 16:15:37 - Failed to get 2XA file from IIS Server (example.com:443).
[E 02/00000037/T01A0/P5DF8/S0001] 01-10-19 16:22:41 - Failed to get 2XA file from IIS Server (example.com:443).
[E 02/00000037/T5758/P1A48/S0001] 01-10-19 16:23:43 - Failed to get 2XA file from IIS Server (example.com:443).
[E 02/00000037/T60F8/P6704/S0001] 01-10-19 16:24:09 - Failed to get 2XA file from IIS Server (example.com:443).

Cause

Requests from web browser and Parallels Client are routed to different gateways by the load balancer.

Resolution

Please reconfigure the load balancer for using session affinity based on cookies.
HTML5 Gateway uses a cookie with ID ASP.NET_SessionId
 

NOTE: Please contact your load balancing solution vendor for detailed configuration.