FortiGate 40F - Web Mode and Angular 9

Hi there, a customer wants to access an Angular 9 based web application through a SSL-VPN portal with enabled web mode. Unfortunately, this doesn't seem to work because the proxy rewrites the deployed Angular application incorrectly. Some injected functions cannot be found, for example.

ERROR TypeError: fgt_sslvpn.host_restore(...) is undefined

The router is broken, too. As you can see in the example below the complete proxy url is added to the url:

https://192.168.41.181/proxy/1f781a4a/http/192.168.41.92/proxy/1f781a4a/http/192.168.41.92/login

Is there any option to make Angular 9 work with web mode? If this is not the case, is it possible to setup a reverse HTTP proxy with authentication in FortiOS? Kind regards, Sebastian