WAF: prevent "403 Forbidden" if Real Webserver down

Hello!

I use Bitwarden (self-hosted) behind the reverse proxy of the UTM (9.707-5).
When I shut down the Bitwarden server (or it is unavailable for some other reason) and I start the Windows client and want to log in, the login does not work because Bitwarden seems to get a "strange" response:

"Forbidden

You don't have permission to access this resource.

Additionally, a 503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request."

The UTM then recognises at "Virtual Webservers" that the server is "in Error" (yellow exclamation mark).

The problem here is that I cannot use the offline functionality (Bitwarden - Client).
If I switch off the "Virtual Webserver" (and thus no answer comes from the UTM), the client works in offline mode.

Here is my question: Can I get the UTM to change this behaviour? So that if the "real web server" is not accessible, the UTM does not return an answer via the "virtual web server", but the request times out?

I will of course also report this to Bitwarden, but I think I will get a quicker answer here....

Thank you!